240 



K. MITSUKURI. 



come to the same conclusion, us a glance at Figs. 18 and 14 will show. 

 Both the surface-views (Figs, ^a ct i^eq.) and the sections give us even an 

 impression that the primitive knoh is spreading itself under the lower 

 layer of the embryonic shield. In the parts where the primitive knob 

 has once established itself, we can, however, no longer distinguish 

 cells that have come from the primitive knob from those of the lower 

 layer of the shield : they are indistinguishably fused. The invagina- 

 tion cavity at this stage (Fig. 14) has much greater longitudinal 

 extension than in that of Fig. 13. I can discover neither at this nor 

 at any subsequent stage a j^osteriorly-directed limb of the invagina- 

 tion-cavity, such as is described by Wenckebach (Xo. 15) in his Fig. 3. 



There is nothing special to say of the nwf of the invagination 

 cavity, except that the points described in the previous stage are all 

 more pronounced in this one. In the floor, there are some im])ortant 

 changes. In the ])osterior half, where the mass of the peristomal 

 mesoblast, grown much more compact, is easily recognisable, there is 

 not much that is new. l)ut in the anterior half of the floor, 

 the wall of the invagination cavity is no longer so sharply 

 defined as before, and some meshes of the cellular network 

 in the primitive knob even open into the invagination- 

 cavity, so that we can here, already in this stage, pass 

 by a labyrinth of intercellular passages from the in- 

 vagination -cavity to the subgerminal yolk -cavity. It 

 should be specially noted that the anterior and of the invagination 

 cavity is distinct and does not share in the dissolution of the anterior 

 part of the floor. 



With the growth of the embryo, the changes in progress between 

 the stao-e of Fi«:. 13 and that of Fio-, 14 become more and more 

 pronounced. The primitive knob grows forwards moi'e and more on 

 the ventral surface of the shield, so that its antero -posterior diameter 



