236 



K. MITSUKURI. 



in laro-e numbers l^ut are scattered, at least near the surface, in- 

 ditterently through the yolk-substance. At different points in the yolk 

 we find unusually large nuclei surrounded by a comparatively small 

 amount of protoplasm (Fig. 11« and h). Sometimes there is only 

 one nucleus (Fig. lib) and then it is very large. The one repre- 

 sented in Fig. ll/> measures .04 x .032 mm. Quite iis often, the 

 nuclei occur in a group of t^^o or three, closely adherent to one 

 another (Fig. 11a). These cells are no doubt what are called 

 " Merocyten " l^y Virchow (No. 14). What their nature is, whether 

 they stand in some genetic relation to other kinds of cells or are of a 

 nature siii generis, I am unable to say. I have thought it just possible 

 from the frequency with which two or three nuclei are found together, 

 that they are cells dividing by amitosis and possibly undergoing dis- 

 integration (Flemnn'ng [No. 4] and Ziegler [No. 20]). 



Let us consider for a moment how such a stage as that described 

 has been reached. What I am inclined to think as probable is as 

 follows : — When the process of segmentation has gone on lor some 

 time, the blastoderm separates itself into two layers, the superficial 

 epiblast and the lower layer. Tliis takes place throughout the blasto- 

 derm with the exception of the primitive plate.* Here 

 cells not only remain undifferentiated but with the addition of cells 

 from the subjacent bed of yolk form a mass which protrudes into the 

 yolk — the primitive kn(j]j. In the middle of this region, an 

 invagination soon appears, which is at first shallow and is directed 

 straight downwards. I have two specimens of this stage but have 

 not figured them because tlie blastoderms having Ijeen peeled off from 

 the yolk to which it is adherent at this stage, the lower part of it is 



* And proljably also of the growing- edge of the blastoderm, hut of this part I aui not now, 



speaking 1 am gratified to find that what is given above as pro)3able is now verified by 



Will by direct observation (See No. 21, Figs. 35 & 36). 



