Blastula

“ Hence the blastoderm, which forms the single-layer wall of the globular blastula at the end of the cleavage-process, does not consist of homogeneous cells of equal size, as in the Sagitta and the Monoxenia; the cells of the upper half of the blastoderm (the mother-cells of the ectoderm) are more numerous and smaller, and the cells of the lower half (the mother-cells of the entoderm) less numerous and larger. Moreover, the segmentation-cavity of the blastula (Fig. 38 C, h) is not quite globular, but forms a flattened spheroid with unequal poles of its vertical axis. ”

“ The morula of most animals further changes into a Blastula, a hollow ball filled with fluid, the wall being formed by a single layer of cells, the blastoderm or germinal layer. This modification is brought about by the action of the cells—they conveying nourishing fluid into the interior of the whole cell colony and thereby being themselves forced towards the surface. The Blastula of most Invertebrata, and even that of Amphioxus, is possessed of fine ciliæ, or hair-like processes, the vibrating motion of which causes the whole organism to rotate and advance in [Pg 51] the water. ”

“ The germ-layer formation is always by immigration or delamination, never by invagination. When the blastula is oval and free-swimming the inner mass is formed by unipolar immigration from the hinder pole. When the blastula is spherical and not set free, the germ-layer formation is always multipolar, either by immigration or by delamination, i.e. by tangential division of the cells of the blastoderm, as in Geryonia, or by a mixture of immigration and delamination, as in Hydra, Tubularia, &c. ”

“ Aside from the question of periblast, the growth of the blastoderm appears not unlike that of the sturgeon. From the blastula stage to that of the early gastrula, the changes have been but slight; the blastoderm has greatly flattened out as its margins grow downward, leaving the segmentation cavity apparent. The rim of the blastoderm has become thickened as the 'germ-ring'; and immediately in front of the dorsal lip of the blastopore its thickening marks the appearance of the embryo. The germ-ring continues to grow downward, and shows more prominently the outline of the embryo ”

“ It is a thin layer of flat epithelial cells, that is freed from the surface of the blastoderm in some of the rodents; it has no more significance in connection with the general course of placental gastrulation than the conspicuous departure from the usual globular shape in the blastula of some of the ungulates. In some pigs and ruminants it grows into a thread-like, long and thin tube.
Thus the gastrulation of the placentals, which diverges most from that of the amphioxus, the primitive form, is reduced to the original type, the invagination of a modified blastula. ”

“ It is important to notice that the formation of the “blastula” from the last cleavage stage is certainly a process of organisation, and may also be called a differentiation with regard to that stage. But there is in the blastula no trace of one part of the germ becoming different with respect to others of its parts. If development were to go on in this direction alone, high organisatory complications might occur: but there would always be only one sort of cells, arranged in a sphere; there would be only one kind of what is called “tissue.” ”

“ The combined force of all these vibrating lashes causes the whole blastula to move about in a rotatory fashion. In many other animals, especially those in which the embryo develops within enclosed membranes, the ciliated cells are only formed at a later stage, or even not formed at all. The blastosphere may grow and expand by the blastodermic cells (at the surface of the sphere) dividing and increasing, and more fluid is secreted in the internal cavity. ”

“ We know that the ovule or primitive genital cell of every animal may be compared to a unicellular organism. After fertilisation the egg undergoes consecutive divisions or segmentation; each segment constitutes a new cell, and their aggregation forms a hollow sphere called a blastula, which is similar to a colony of unicellular beings. The blastula differentiates itself into embryonic layers, the ectoderm, endoderm, and mesoderm already mentioned.
In the majority of animals the origin of the first two layers, ectoderm and endoderm, is due to the invagination of one of the poles of the blastula ”

“ The cells which in the above manner have entered the interior of the blastula are to be the foundation of important parts of the future organism; they are to form its connective tissue, many of its muscles, and the skeleton. “Mesenchyme,” i.e. “what has been infused into the other parts,” is the technical name usually applied to these cells. We now have to learn their definite arrangement. At first they lie as a sort of heap inside the cell wall of the blastula, inside the “blastoderm,” i.e. skin of the germ. ”

“ We still find, both in the sea and in fresh water, various kinds of primitive multicellular organisms that substantially resemble the blastula in structure, and may be regarded in a sense as permanent blastula-forms—hollow vesicles or gelatinous balls, with a wall composed of a single layer of ciliated homogeneous cells. ”

“ We may add that the swimming blastula, consisting of about one thousand cells, when cut in two quite at random, in a plane coincident with, or at least passing near, its polar axis, may form two fully developed organisms out of its halves.25 We may formulate this result in the words: the prospective potency of the single cells of a blastula of Echinus is the same for all of them; their prospective value is as far as possible from being constant. ”

“ In cases where there is no yolk, or very little, as in Lucifer and Amphioxus respectively, the embryo is compelled to feed itself at a very early age; such embryos form a free-swimming pelagic ciliated blastula, the invagination of which, for the purpose of collecting food material out of the open sea, is the simplest method of obtaining nutriment. ”

“ In the evolution of these embryos with which there is not laid up a large amount of food-yolk there occurs at the outset a striking omission of this kind. When, by successive fissions, the fertilized cell has given rise to a cluster of cells constituting a hollow sphere, known as a blastula, the next change under its original form is the introversion of one side, so as to produce two layers in place of one. ”

“ Thus in Clathrina blanca the ovum undergoes a regular and total cleavage, resulting in the formation of a hollow ciliated blastula of oval form. At one point, the future posterior pole of the larva, are a pair of very large granular cells with vesicular nuclei, which represent undifferentiated blastomeres and are destined to give rise to the archæocytes, and, therefore, also to the {474}sexual cells of the adult. Thus, as he says, from the very earliest period a distinction is made between the "tissue-forming" cells (my syncytial host) and the archæocytes. ”

“ If the single phases of differentiation are to be regarded as effects, we must ask for the causes, or stimuli, of these effects. For a full account of the subject we refer to Herbst, by whom also the whole botanical literature, much more Formative stimuli. important than the zoological, is critically reviewed. We have already seen that when the blastula represents an aequipotential system, there must be some sort of primary organization of the egg, recoverable after disturbances, that directs and localizes the formation of the first embryonic organs ”

“ In this form the segmentation results in a uniform, or nearly uniform, blastosphere, one wall of which soon becomes thickened and invaginated, giving rise to the hypoblast; while the larva takes the form of a gastrula, with an archenteric cavity opening by a blastopore. The blastopore rapidly narrows, while the [Pg 276] embryo assumes an elongated cylindrical form with the blastopore at its hinder extremity (fig. 169 A) . The blastopore now passes to the dorsal surface, and by the flattening of this surface a medullary plate is formed extending forwards from the blastopore (fig. 169 B) . ”

“ The food-yolk in the Vertebrata is placed on the ventral side of the body, and becomes enveloped by the blastoderm; so that in all large-yolked Vertebrates the ventral walls of the body are obviously completed by the closure of the lips of the blastopore, on the ventral side.
If His and Rauber are right the dorsal walls are also completed by the closure of the blastopore, so that the whole of the dorsal, as well as of the ventral wall of the embryo, must be formed by the concrescence of the lips of the blastopore; which is clearly a reductio ad absurdum of the whole theory. ”

“ This stage is shown in section in the lower figure of Figure 1. b.d., the blastoderm, is from this point of view, a part of the ripped and flattened blastosphere, spread out on the yolk; s.c. is the segmentation cavity, and y. the yolk.
Section 25. There is no open invagination of an archenteron in the fowl, as in the frog--, the gastrula, like the blastosphere, stage is also masked. But, in the hinder region of the germinal area, a thick mass of cells, grows inward and forward, and, appearing in the dorsal view of the egg as a white streak, is called the primitive streak (p.s.) . ”

“ The eight-celled stage (fig. 38, b, c,) has the form of a somewhat flattened cushion, with an axial cavity which is the beginning of the blastocoel or segmentation cavity. A horizontal cleft now divides each blastomere into a somewhat smaller upper and a somewhat larger lower portion, and the sixteen blastomeres arrange themselves in the form of a hollow sphere surrounding the blastocoel. The smaller cells multiply rapidly and become columnar, while still remaining as a single layer. Each one presently acquires a flagellum (“cilium”) at its outer end. ”

“ There is one point to be noticed in the shape of the blastoderm as a whole. It is unsymmetrical, and a much larger number of its cells are found collected at one end than at the other. This absence of symmetry is found in all sections which are cut parallel to the long axis of the egg-capsule. The thicker end is the region where the embryo will subsequently appear. ”

“ In a second class of cases, the Morula becomes converted into blastomeres of unequal sizes, a small and a large set. The smaller rapidly become converted into cells, and invest the larger and any remains of the food yelk, as a blastoderm, which at first represents only the epiblast of the former case. The hypoblast arises either from the epiblast thus formed, or from the included larger blastomeres. This is the process observed in certain Turbellaria, in the Ctenophora, in the Oligochoeta and Hirudinea, in the Artliropoda, and in most Vertebrata. ”

“ The phenomenon is one of considerable interest, for it is found in the case of the blastopore, in cases in which there is no metamorphosis, sometimes even in embryonic development. There can be little doubt that the mouth and anus are both genetically connected with the earlier blastopore and that the blastopore is homologous in most animals; and yet how seldom does the blastopore become transformed into the adult openings and how various is its fate. ”