20 K. MTTSUKURI AND C. ISHIKAWA. 



surface again, the area behind the blastopore, especially the 

 median longitudinal space, is on a lower level than the parts in 

 front. This, the sections show us, is the line of the primitive 

 streak. At the part where the embryonic shield posteriorly 

 joins the area opaca there is a considerable transverse thicken- 

 ing (si.), shown both in the dorsal and ventral views — in the 

 latter covered with yolk matter. This undoubtedly corresponds 

 to the "sichel" or "sickle" which Kupffer describes in a 

 similar Lacerta embryo (No. 5, Taf. i, fig. 1, si.). We should 

 add that these diflPerences in level become much more con- 

 spicuous after the embryonic shield has been removed and 

 treated in reagents than when it is stretched over the yolk, 

 and also that the embryos of this stage vary considerably in 

 their surface views, especially when they are hardened. 



Figs. 7 — 15 are selected from the series of transverse sec- 

 tions obtained from the embryo represented in figs. 1 a and b. 

 The figures are arranged in order from behind forward. Figs. 

 7 — 9 pass through the part behind the blastopore, figs. 10, 11 

 through the blastoporic passage, and figs. 12 — 15 through the 

 part in front of the blastopore. 



In fig. 7, the most posterior section represented, the ecto- 

 blast extends over the whole, being two or three layers of cells 

 thick in the embryonic shield, but gradually thinning out to a 

 single layer of flat cells toward both sides. The yolk occupies 

 the entire lower stratum. Nuclei {n. n.) are visible in it. The 

 space between the ectoblast and the yolk is occupied by a mass 

 of mesoblast cells which is here distinctly separate from both 

 the ectoblast and the yolk. 



In fig. 8 (which by the way is taken from another embryo of 

 the same deposit, as the section corresponding to this in the 

 first series is unfortunately injured) the ectoblast is continuous 

 in the median line with the mesoblast, i. e. it is very actively 

 proliferating and giving off cells abundantly to the mesoblast. 



Fig. 9 passes through the region directly behind the blasto- 

 pore. The ectoblast is distinct laterally, but toward the 

 median line, and at some distance from it, passes gradually into 

 a mass of cells in which no layers can be distinguished. Dif- 



