^^w K. MITSUKUEI AND C. ISHIKAWA. 



opening of the blastoporic passage (compare fig. 26). Conforming to 

 the. groove in the ectoblast, the chorda-entoblast projects downwards 

 in the median line. This corresponds no doubt to the ridge seen in 

 the sm-face view within the lower opening of the blastoporic passage. 

 This section also shows that the roof of the well-defined area which 

 forms the lower opening of the blastoporic passage is formed by the 

 chorda-entoblast and that the latter thus occupies by itself a special 

 recess of the digestive cavity. From just Avliere the chorda- 

 entoblast and the darm-entoblast join each other, there 

 goes out laterally on each side a string of cells (mes.) 

 placed dorsally to the darm-entoblast and ventrally to 

 the ectoblast and distinct from both. This is the com- 

 mencing mesoblast. The mesoblast is therefore not continuous 

 from the first across the median line. 



The surface view of the next stage is represented in fig. 3. 

 The embryonic shield has become pear-shaped, the broader end being 

 the front end. The blastopore is horseshoe shaped, as in figs. 2 a 

 and h, and occupying its concavity is the yolk-plug. The head-fold 

 has just begun, and is seen as the posterior of the two semilunar 

 carves found near the front end of the embryonic shield. The 

 anterior curve is probably the commencing amniotic fold. Between 

 tlie head-fold and the blastopore there is seen in the median line an 

 opaque streak, which is narrowest in the middle, and becomes broader 

 anteriorly and, posteriorly. This is the chorda, which is nearly 

 completed in the middle, but still unfinished toward. each end. The 

 area pellucida is, as before, found only toward the front and the sides. 

 The pear-shape of the embryonic shield seems to have been produced 

 mainly by its posterior part having lengthened. 



Figs. 18- — 23 are selected from a series of cross-sections obtained 

 from an embryo of this stage, and are arranged from behind forwards. 



