42 AKT. 11- — S. HATTA. 



(6. y.) has öliil'ted far anteriorly, and the segmentation cavity (.s'.c.) 

 is very much reduced. Some disturbed macromeres (««-) are seen 

 outside the blind end of the archenteric canal, x 80. 



Plate III. 



Fig. 20 — A median sagittal section through an ovum a little further 

 advanced than that shown in surface view in Fig. 9«. The 

 archenteron [ar.cn.) has further extended, this extension being 

 made by further invagination of the macromeres as well as by 

 new addition of micromeric epithelium. The segmentation 

 cavity [s.c.) is reduced into a narrow space ; the boundary groove 

 {h.g.) is marked by a slight depression on the ventral surface of 

 the last remnant of the segmemtation cavity, which is about to 

 be obliterated by the falling in of the macromeres loosened from 

 mutual union, x SO. 



Fig. 21 — A median sagittal section through an ovum of about the same 

 development as the surface view given in Fig. 10«. Tiie arch- 

 enteron (ar.en.) is much increased in extent ; its anterior part 

 strikes against the peristomal part of the mesoderm (p.m.) now 

 formed, thus entirely obliterating the segmentation cavity. The 

 micromeric section of the archenteric roof has greatly enlarged 

 so that nearly two-thirds of the latter is taken up by the micro- 

 meric epithelium (^i.mic.ep.) ; a small part of the macromeric 

 field still remains uninvaginated. The strongly wrinkled chorion 

 [cli.) is shown, x 80. 



Fig. 22 — A median sagittal section of a nnich advanced ovum which is, 

 however, a little j'Ounger than the ovum figured in Fig. l\a. 

 The process of gastrulation is finished. There is no trace of the 

 macromeric field outside. The outer {e.mic.ep.) as well as the 

 inner {i.mic.ep.) micromeric epithelia show a compact texture. 

 The anterior part of the archenteron is lost from sight ; the cells 

 which formerly formed the wall at this part are now indistinguish- 

 able from other macromeric elements which have lost their nuitual 

 union. The peristomal mesoderm [p.m.) is well established. 

 xSO. 



Fiq. 23 — A piece of a sagittal section through an ovum a little yt)nnger 

 than that represented in Fig. 14. The jeft side is the outer 

 surface, and the right represents the surface turned towards the 



