400 MEMOIRS OF TEE NATIONAL ACADEMY OF SCIENCES. 



before all the protoplasm, that is, tlie nuclei and perinuclear protoplasm of tlie yolk pyramids, 

 has reached the surface. In the slightly older idiase, shown in PI. xxx, Fig. 4«, all the proto- 

 l)lasm which does not pass inward is strictly superficial. Tlie yolk has the same appearance as 

 in previous stages, and, as already noticed, the cleavage i)lanes (Sep.) between yolk pyramids are 

 still met with. Very soon, however, the central portion of the yolk segments into biills or angular 

 blocks (Fig. 46, Y. B.), apparently with reference to these wandering cells. A section through the 

 germinal disk of an egg seven and one-half hours older than that shown in Fig. 47 is given in Fig. 

 57. The cells in the area of the germinal disk are (piite closely crowded, and the superticial seg- 

 mentation of the yolk is still apparent. We now have a primitive epiblast, or external layer of 

 cells, and a primitive hypoblast, composed of yolk <'ells which have migrated from the blastoderm. 



The invagination stage immediately follows that last described (PI. xxxi). A slight depres- 

 iou occurs at a point on one side of the germinal disk, where the cells are multiplying most 

 rapidly, and numerous cells pass downward into the yolk. The invagination is nearly solid, and 

 the segmentation cavity is still filled with the great mass of yolk and with primitive hypoblast. 

 In the crayfish (Astacws fluviatilis) the invaginate cavity becomes a closed chamber within the 

 yolk (54), and this is eventually converted into the midgut, but in most decapods the pit is very 

 small and the mesenteron is formed independently at a later period. A line drawn through the 

 pit and the middle of the germinal disk marks the long axis of the embryo, and the point of 

 ingrowth is at the posterior end. 



The structure of the embryo is illustrated by a series of transverse sections (PI. xxxi). The 

 cells in the center of the egg represent the primitive hyjjoblast or yolk cells. The nuclei are 

 large and granular, and sometimes occupy the center of a yolk ball. In Fig. 4!) the posterior edge 

 of the embrjo is sectioned, and the three following sections (Figs. 50-53) pass through the region 

 corresponding to the invaginate area (Igf). Fig. .52 represents the entire "gectiou, of which Fig. 

 51 is a part. The pyrami'dal cells, which form the lloor of the depression, contain at their 

 peripheral ends no unabsorbed yolk, but at the deeper ends of the cell, below the level of the 

 nucleus, the cell boundaries are lost, and the protoplasm of the cell blends off into the yolk and 

 ingulfs its finely divided i)articles (Fig. 50). Numerous cells (Figs. 52-54, b, b^ =) have already 

 wandered from the point of invagination into the egg and a considerable distance forward under 

 the germinal disk (Fig. 54, G. D.). These cells are more or less intimately united by pseudopodal 

 extensions of the protoplasm. A coarse reticulum is thus formed, the meshes of which are filled 

 with yolk. In front of the invaginate cells, the germinal disk (Fig. 55, G. D.) is still one cell 

 thick. At the close of the invagination stage the primitive hypoblast has received a consid- 

 erable accsssion of wandering cells. This .stage is usually described as the "egg-gastrula," 

 in accordance with the theory that it represents an ancestral condition, and that the cavity 

 formed at the surface is the remnant of the primitive digestive tract. The discovery of delami- 

 nation, the actual separation of the inner ends of the cells of the blastula by karyokinesis before 

 any invagination occurs, as I have described in the lobster, and the occurrence of this or of mul- 

 tipolar emigration in Alpheus, together with the fact that in the typical decapod the invagination 

 has no direct relation to the digestive tract or to to the mouth and anus, point to the view already 

 expressed in a preliminary paper upon the lobster (25), that the invagination stage has no refer- 

 ence to an ancestral invaginate gastrula. It seems to me more probable that the egg with primary 

 yolk cells corresponds to the ccelenterate plauula stage, and that these yolk cells, which originate 

 from the blastula and which i)artially or entirely degenerate, represent the remains of a i)rimitive 

 hypoblast. According to this view the invagination is a secondary process, which became so 

 indelibly impressed upon the ancestors of the Decapods that it has remained in the ontogeny of 

 present forms. The conditions which are found in the crayfish can not be regarded as in any 

 sense general or typical. 



Stage III. — Optic disks and ventral plate formed. 



This stage (Fig. 58) is characterized by a thickening of epiblast which gives rise to the tho- 

 racic-abdominal or ventral plate in front of and around the point of ingrowth, by the simultaneous 

 spreading of invaginate cells below the surface, and by the appearance of the optic disks (O. D.), 



