In such a case as this, yelk division is said to be complete. An unessential modification of complete yelk division is seen when, at an early period, the blastomeres produced by division are of unequal sizes; or when they become unequal in consequence of division taking place much more rapidly in one set than in another.
In many animals, especially those which have large ova, the inequality of division is pushed so far that only a portion of the yelk is affected by the process of fission, while the rest serves merely as food-yelk, for nutriment to the blastomeres thus produced. Over a greater or less extent of the surface of the egg, the protoplasmic substance of the yelk segregates itself from the rest, and, constituting a germinal layer, breaks up into the blastomeres, which multiply at the expense of the {207} food-yelk, and fabricate the body of the embryo. This process is termed partial or incomplete yelk division.
The crayfish is one of those animals in the egg of which the yelk undergoes partial division. The first steps of the process have not yet been thoroughly worked out, but their result is seen in ova which have been but a short time laid (fig. [57], A). In such eggs, the great mass of the substance of the vitellus is destined to play the part of food-yelk; and it is disposed in conical masses, which radiate from a central spheroidal portion to the periphery of the yelk (v). Corresponding with the base of each cone, there is a clear protoplasmic plate, which contains a nucleus; and as these bodies are all in contact by their edges, they form a complete, though thin, investment to the food-yelk. This is termed the blastoderm (bl).
Each nucleated protoplasmic plate adheres firmly to the corresponding cone of granular food-yelk, and, in all probability, the two together represent a blastomere; but, as the cones only indirectly subserve the growth of the embryo, while the nucleated peripheral plates form an independent spherical sac, out of which the body of the young crayfish is gradually fashioned, it will be convenient to deal with the latter separately.
FIG. 57.—Astacus fluviatilis.—Diagrammatic sections of embryos; partly after Reichenbach, partly original (× 20). A. An ovum in which the blastoderm is just formed. B. An ovum in which the invagination of the blastoderm to constitute the hypoblast or rudiment of the mid-gut has taken place. (This nearly answers to the stage represented in fig. [58], A.) C. A longitudinal section of an ovum, in which the rudiments of the abdomen, of the hind-gut, and of the fore-gut have appeared. (This nearly answers to the stage represented in fig. [58], E.) D. A similar section of an embryo in nearly the same stage of development as that represented in C, fig. [59]. E. An embryo just hatched, in longitudinal section; a, anus; bl, blastoderm; bp, blastopore; e, eye; ep. b., epiblast; fg, fore-gut; fg1, its œsophageal, and fg2, its gastric portion; h, heart; hg, hind-gut; m, mouth; mg, hypoblast, archenteron, or mid-gut; v, yelk. The dotted portions in D and E represent the nervous system.
Thus, at this period, the body of the developing crayfish is nothing but a spherical bag, the thin walls of which are composed of a single layer of nucleated cells, while its cavity is filled with food-yelk. The first modification {209} which is effected in the vesicular blastoderm manifests itself on that face of it which is turned towards the pedicle of the egg. Here the layer of cells becomes thickened throughout an oval area about 1‐25th of an inch in diameter. Hence, when the egg is viewed by reflected light, a whitish patch of corresponding form and size appears in this region. This may be termed the germinal disk. Its long axis corresponds with that of the future crayfish.
Next, a depression (fig. [58], A, bp) appears in the hinder third of the germinal disk, in consequence of this part of the blastoderm growing inwards, and thus giving rise to a small wide-mouthed pouch, which projects into the food-yelk with which the cavity of the blastoderm is filled (fig. [57], B, mg). As this infolding, or invagination of the blastoderm, goes on, the pouch thus produced increases, while its external opening, termed the blastopore (fig. [57], B, and 58, A–E, bp), diminishes in size. Thus the body of the embryo crayfish, from being a simple bag becomes a double bag, such as might be produced by pushing in the wall of an incompletely distended india-rubber ball with the finger. And, in this case, if the interior of the bag contained porridge, the latter would very fairly represent the food-yelk.
