214 K, MITSUKURI AND C. ISHIKAWA. 



ventral side we see that the two lateral opaque lines correspond to the 

 thickenings which from the walls of the inferior opening of the 

 blastoporic passage. Accordingly they are thickest posteriorly, and 

 gradually thin out toward the front. The middle prong of the trident 

 corresponds to the roof of the blastoporic passage and its continuation 

 to the front edge of the embryonic shield. It is, in fact, the chorda 

 entoblast, which is still in the process of formation in front, as 

 will be made clear by sections. The remaining parts of the ventral 

 surface not taken up by these three thickenings present the appearance 

 of a honeycomb. Of this we shall speak later on. Coming back to the 

 dorsal 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 thickening (sL), shown both in the dorsal 

 and ventral views — in the latter covered with yolk matter. This 

 undoubtedly corresponds to the "sichel" or "sickle" which Kuptfer 

 describes in a similar Lacerta embryo (Ko. 5, Taf. i, fig. 1, d.). We 

 should add that these differences 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, espe- 

 cially when they are hardened. 



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

 obtained from the embryo represented in figs. 1 a and h. The figures 

 are arranged in order from behind forward. Figs. 7 — 9 pass through 

 the part behind the blastopore, figs. 10, 1 1 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 ectoblast 

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



