The simplicity of these first steps, whether it points to primitiveness or not, affords a view of certain fundamental principles of development which makes the study of higher vertebrate forms much easier and renders their formative processes much more intelligible. This simplicity is probably correlated with the freedom of the egg from a large amount of yolk; and it will be seen that many of the modifications of the processes of development in the vertebrates seem to be produced by the greater amount of yolk in their ova. Cleavage The ovum of Amphioxus has certain peculiarities which are important in their effect upon cleavage. While it contains only a small quantity of yolk, being regarded as a meiolecithal ovum, this material is situated slightly off center and the nucleus lies outside of the yolk Fig. This condition really effects a polarity of the cell.
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The simplicity of these first steps, whether it points to primitiveness or not, affords a view of certain fundamental principles of development which makes the study of higher vertebrate forms much easier and renders their formative processes much more intelligible.
This simplicity is probably correlated with the freedom of the egg from a large amount of yolk; and it will be seen that many of the modifications of the processes of development in the vertebrates seem to be produced by the greater amount of yolk in their ova. Cleavage The ovum of Amphioxus has certain peculiarities which are important in their effect upon cleavage.
While it contains only a small quantity of yolk, being regarded as a meiolecithal ovum, this material is situated slightly off center and the nucleus lies outside of the yolk Fig. This condition really effects a polarity of the cell.
The first polar body is given off from the yolk-free portion of the egg. This marks the animal pole and also the side which will be the anterior part of the embryo. The sperm enters the egg at the vegetative pole and seems to stimulate the formation of the second polar body. The sperm nucleus and centrosome then traverse the yolk area to meet the mature egg nucleus which in the meantime has migrated toward, but not quite to, the center of the egg.
The division of the sperm centrosome to form a disaster and the arrangement of the chromosomes of the two pronuclei in the equatorial plane comprise the preparatory step for the first cleavage.
These phenomena are identical with the prophase of mitosis Fig. Diagram of a median sagittal section through an Amphioxus ovum. Cerfontaine, from Kellicott. The arrow indicates the direction of the polar axis. AD, antero-dorsal region; PV, postero-ventral region; N, male and female pronuclei; p, yolk-free area; S, tail of sperm; y, yolk area; II, second polar body.
The position that the spindle assumes is determined by three factors: the point where the first polar body is extruded, the point where the sperm enters, and the location of the yolk-free area. A plane bisecting this area and passing through the other two points will divide the egg into symmetrical halves. The spindle takes its position at right angles to this plane. The first cleavage therefore will produce two equal and symmetrical daughter cells, or blastomeres, the first cleavage plane coinciding with the plane of symmetry of the ovum.
These two blastomeres will become the right and left halves of the embryo, the plane of symmetry of the ovum representing the sagittal plane of the embryo. With the anterior portion already indicated by the point of extrusion of the first polar body, the orientation of the first two blastomeres relative to the future embryo is now complete.
Prophase of first cleavage figure in the ovum of Amphioxus. The chromosomes of the male and female pronuclei are mingled in the equatorial plane. Sobotta, from Kellicott. The second cleavage plane falls at a right angle to the first, cutting both the animal and the vegetative pole. The division is slightly unequal, however, the result being two slightly smaller blastomeres and two slightly larger blastomeres Fig. These are arranged symmetrically on the two sides of the median plane.
The third cleavage plane lies at right angles to the other two, and division of the cells is again slightly unequal a condition often called subequal , the result being four pairs of cells of four different sizes Fig. The smallest cells are those derived from the portion of the ovum which contained less yolk, the largest are those derived from the portion which contained more yolk.
All the cells have divided completely, a circumstance which gives rise to the term total cleavage; and this condition obtains throughout the later stages. All the cells at a given cleavage thus far have divided at the same time, a fact which is expressed in the term regular cleavage. If cleavage were to continue regularly the result at succeeding divisions would be 16, 32, 64, cells, and so on. Regularity is lost, however, during the fourth cleavage, some of the cells dividing before others, with the result that numbers other than those just given will be found.
The smallest cells, with the least amount of yolk are the first to divide and they divide more rapidly than the large cells with a greater yolk content; the inert non-protoplasmic substance retards the progress of division. Cleavage in Amphioxus. A, four-cell stage seen from animal pole; B, eight-cell stage seen from animal pole, showing four sizes of blastomeres; C, sixteen-cell stage seen from left side; A, thirty-two-cell stage seen from vegetal pole; E, cells seen from antero-dorsal region; F, half of early blastula containing about cells, a, Animal pole; ad, antero-dorsal; I, left; pv, postero-ventral; r, right; v, vegetal pole.
Division succeeds division in the blastomeres, with the irregularity noted in the preceding paragraph. The cleavage planes vary considerably in direction in different individuals. At the i6-cell stage the micromere group assumes a sort of dome form and the macromere group in similar form fits into the hollow of the dome Fig.
The early blastomeres remain well rounded so that even at the four-cell stage there is a small central cavity Fig. As cleavage progresses the cells become more closely arranged and pushed away from the central cavity Fig. At the icell stage all the cells are arranged in a simple epithelial layer around a relatively large central cavity, the segmentation cavity or Uastoccel. The entire structure is now the bias tula.
Other divisions occur until the blastula contains about cells. There is a gradual transition from the micromeres at one pole of the hollow sphere to the macromeres at the opposite pole. It should be recalled here that, on account of the position of the yolk-free portion of the ovum, the micromeres lie where the anterior region of the embryonic body will arise and the macromeres where the posterior region will develop.
About four hours elapse between the time the first cleavage occurs and the time the cell blastula is formed. Gastrulation This process comprises the conversion of the single walled blastula into the double walled gastrula.
The vegetative pole becomes flattened, the macromeres assuming columnar form. The cells at the dorsal margin of the flattened pole begin to proliferate more rapidly than elsewhere, as shown by the increased number of mitotic figures Fig. This area of accelerated division then extends in both directions around the margin of the flat pole, forming the germ ring. Beginning at the dorsal margin the macromeres are folded, or invaginated, into the blastocoel until the blastoccel is obliterated Fig.
A rough analogy is the pushing in of one side of a hollow rubber ball. The invagination, however, is more rapid along the dorsal margin of the plate of macromeres, and as the infolding progresses there is formed a plate of small cells which arise through the more rapid proliferation in the germ ring Fig.
On the ventral side the ingrowth is but slight, the whole plate of macromeres behaving as if hinged at this point. By these processes the blastula, with a single layer of cells, has been converted into the gastrula, with a double layer of cells and a new cavity which opens to the exterior.
The outer layer of cells is the ectoderm which is in direct contact with the environment of the developing organism. The inner layer is the entoderm which forms the lining of the new cavity, or archenteron, in the interior of the organism. The entoderm consists of two types of cells, the larger cells with considerable yolk content which lie on the ventral side or in the floor of the archenteron and the smaller cells forming the dorsal lining of the archenteron which were produced by the rapid divisions in the germ ring.
This latter group in part really had a brief existence as ectodermal cells and then contributed to entoderm by being inflected round the rim of the opening between the archenteron and the exterior. The inflection of the cells in question, often called involution is therefore one of the factors in gastrulation. The circular opening between the archenteron and the exterior is the blastopore. Its margins are its lips which can be differentiated into dorsal, ventral and lateral lips.
At these lips the entoderm and ectoderm are continuous. Another factor in gastrulation is a process known as epiboly. When invagination is complete, that is, when the macromere pole of the blastula has infolded until the blastoccel is obliterated, the gastrula approximates a hemisphere and the form of the archenteron coincides.
Then, along with the rapid cell proliferation in the dorsal part or the germ ring and the formation of the plate of entodermal cells mentioned in the preceding paragraph, the dorsal lip of the blastopore extends backward. The lip protrudes, one might say. The extension gradually affects also the lateral lips and finally to a slight degree the ventral lip.
This whole process of growth backward, which is due to the rapid cell division in the germ ring most rapid dorsally, less rapid laterally, least rapid ventrally, effects a posterior elongation of the gastrula and a diminution in the size of the blastopore Fig. This is the first step in the lengthwise growth of the animal as a whole. The whole process of gastrulation has occupied about three hours.
Gastrulation in Amphioxus. A, blastula with slightly flattened vegetal pole, showing rapid cell division in postero-dorsal region germ ring ; 5, more pronounced flattening of the vegetal pole; C, beginning of invagination in postero-dorsal region; D, further invagination, showing obliteration of the blastocoel and formation of the archenteron as the result of invagination; E, invagination almost complete; F, beginning elongation of gastrula and narrowing of blastopore; G, continued elongation of gastrula and narrowing of blastopore.
Observe the mitotic figures in the germ ring in all stages. In D and E the inflection of cells involution around the dorsal lip of the blastopore can be appreciated. In F and G the process of epiboly is represented in the backward growth of the lip of the blastopore. The account here given differs in one respect from that of the British investigator, MacBride. It has been stated that inflection, or involution, is one of the factors in gastrulation.
MacBride maintains that involution does not occur, but that the rapid cell division occurring in the lips of the blastopore produces both ectoderm and entoderm in equal amounts.
Cell proliferation is the only process which adds to the number of entodermal as well as of ectodermal components, and this at the same time produces the backward extension of the lips of the blastopore which is recognized as epiboly.
He bases his conclusion on nuclear characters. In the bias tula all the nuclei are vesicular. Soon after gastrulation begins the nuclei of the ectodermal cells become more intensely stainable while those of the entodermal cells retain their vesicular nature, all the invaginated cells possessing the vesicular nuclei. This probably indicates a physiological differentiation.
In the germ ring two types of the rapidly dividing cells can be distinguished, one with vesicular nuclei and the other with deeply staining nuclei. The former are added to the entoderm, the latter to the ectoderm. There is therefore a zone of growth in which cells are produced and added directly to the two layers without inflection round the lip of the blastopore. The gastrula is now somewhat elongated antero-posteriorly, somewhat flattened on the dorsal side and is bilaterally symmetrical, with the archenteron opening to the exterior at the caudal end through the small blastopore Fig.
Even at this time it is not amiss to note a certain fundamental arrangement of structure and anticipate in a measure its biological significance when carried over into later stages. The ectoderm, the outer layer of the gastrula, is in immediate contact with the environment, which fact implies that response to external stimuli and protection are effected through this layer.
In Amphioxus, as well as in certain other lower forms, strong cilia develop on the ectodermal cells by the motion of which the gastrula changes its position. In later stages it will be seen that the nervous sytem, that complex mechanism for transmitting stimuli from one part of the body to another, is developed from ectoderm.
The outer layer of the integumentary system with certain of its derivatives, primarily protective in nature, is also a product of ectoderm. The archenteron with its lining of entoderm constitutes the primitive gut, the only opening of which is the blastopore, serving as both mouth and anus.
Already the simple alimentary system is confined to the interior of the organism, shut off from the outside except through an opening for the intake of food and output of waste.
Among the invertebrates the sponges and corals never develop beyond the two layered, or didermic, gastrula stage such as we here see in Amphioxus. It is worth noting also that in Amphioxus the cells with yolk content are members of the entoderm group; in other words, a temporary food supply, scanty as it is here, is stored in the lining of the gut. From this simple primitive gut the whole alimentary system is elaborated, complex as it may become.
Book - Text-Book of Embryology 4
Habitat[ edit ] Lancelets are distributed in shallow subtidal sand flats in temperate as far north as Norway  , subtropical and tropical seas around the world. They live in sandy bottoms whose granulometry depends on the species and the site,  and they are usually found half-buried in sand. Adults B. Branchiostoma floridae is capable of trapping particles from microbial to small phytoplankton size,  while B.
Stage of Development: Embryo Object Description Embryos of amphioxus Branchiostoma floridae from blastula to early neurula. Animal pole is at the left. Amphioxus are marine chordates. They are the earliest diverging subphyla in the Phylum Chordata and thus hold clues to the origins of vertebrates. Amphioxus embryos gastrulate by simple invagination. At the late blastula stage 2. At the onset of gastrulation 3.
Development of Branchiostoma | Cephalochordata | Chordata | Zoology