':38 K. MITSUKURI. 



ß.t the hindmost part, the epiblast is already formed as far hack as is 

 given in the figure. As we trace it forward, it is distinct up to the 

 blastoporic passage, but here it becomes merged in a large thick mass 

 of cells whicli stretches jilong the floor of that passage. The lower 

 layer, which is continuous behind with the yolk-bed and which is 

 distinguished by containing a larger quantity of yolk granules, is also 

 joined to the large mass on the floor of the blastoporic passage by 

 irregular reticulate strands of cells. Thus, behind the blastoporic 

 passage the layers are fused together, and there can be no doubt that 

 the thick mass of cells is the beginning of what Eabl calls the per- 

 istomal mesoblast.* Eventually it will stretch backward for the space 

 of 180° like an open fan, between the epiblast and the definitive hypo- 

 blast. 



Figs. 6-10 are a series of cross-sections from another embryo in the 

 same stage, arranged from behind forward. They confirm what we 

 have learned from the longitudinal section and give us some additional 

 information. 



In Fig. 6, taken from directly behind the blastoporic passage, the 

 epiblast is marked by a groove at about the median line, and is at this 

 point proliferating downwards cells which form a small mass con- 

 tinuous with the lower layer by loose strands of the cells. This pro- 

 liferation is seen more or less distinctly in several c(jnsecutive sections 

 until it joins in front the large mass on the floor of the blastoporic 

 passage. This median liiic of proliferation I take to be the commencing 

 primitive streak. This becomes broader îind more conspicuous in later 

 stages. The same thing was seen in Trionyx (Joe. cit., Figs. 6 

 and 8). 



In Fig. 7, the epiblast is no longer distinguishable in the median 

 region, Avhich is occupied by the solid thick mass of cells already 



* C. Ra,bl :-^Theorie des Mesoderms. Morph. Jahrbuch, 1889. 



