216 K. MITSUKUW AND C. ISHIKAWA. 



connect the two. 



Fig. 10 passes just in front of the dorsal lip of the blastopore 

 where the ectoblast reflects downward and forward to become con- 

 tinuous with the axial strip of the entoblast or chorda- en toblast 

 (compare fig. 16). The blastoporic passage (/;/. j).) seen as a trans- 

 verse space is still open on the left to the exterior. The floor of the 

 passage is formed by a mass of cells continuous with the yolk-plug; 

 in fact we may consider this a part of the plug. At a loAver level the 

 mesoblast (mes.) stretches out laterally as two Avings from the median 

 mass. The relations of the yolk are the same as in fig. 9. 



So far the sections seem to have passed through the part known 

 as the " sickle." 



The next section represented (fig. 11) evidently passes through 

 what may be called the neck or isthmus, i. e. the point from wdiich 

 the three prongs of the trident referred to in the surface view radiate 

 (compare fig. 1 h). Accordingly, the entoblast is found only in the 

 median line as a thickening constituting the walls of the blastoporic 

 passage (hi. p.), Avhich is now irregularly circular in section. The 

 roof and the sides of the passage are formed by a columnar epithelium 

 two or three cells thick (enc). This is continuous with the ectoblast 

 at the dorsal lip of the blastopore (compare figs. 16 and 10). It is 

 the chorda-entoljlast of Hertwig (No. 6). The floor of the passage 

 and the lower part in general is made up of irregularly scattered cells. 

 This is not only the continuation of the yolk-plug but also of the yolk 

 itself, which occupied the lowest stratum in figs. 7 — 10, and which 

 has been in the last two or three sections gradually merging itself 

 into the floor of the blastoporic passage. Thus, although it does not 

 appear in any single transverse section, the three germinal layers are 

 fused in (he region behind the blostopore. Laterally the entoblast is 

 very thin and passes gradually into the yolk. The section is out of 



