18 ART. 4. N. YATSU I 



spherules according to the usual mode of plasmolyses, but some- 

 times they elongate so as to assume the form of a thread, reaching 

 10-15 times their original length. 



VI. GASTRULA UP TO THE OLDEST EMBRYO THAT COULD 

 BE REARED FROM THE EGG. 



At the time when the section of a blastula consists of 30-40 

 cells an invagination commences at the pole where there are taller 

 cells (PI. L, Figs. 12, 13. ; PI. IL, Fig. 33.). As the invagina- 

 tion goes on, the typical gastrula is formed. Now we see in a 

 sagittal section that, as the result of the unequal growth of the 

 invaginated area, the arms of the V-shaped entoblast are not 

 equal in length, so that the tip of the entoblast is directed nearly 

 horizontally (PI. I., Fig. 13.). The mesoblast is rapidly proli- 

 ferated from the entoblast walls (PI. IV., Fig. 54.). The mesoblast 

 cells, however, do not loosen themselves, but adhere closely to the 

 entoblast, forming a compact cell-mass with the latter, which 

 will be called hereafter the mes-entoblastic cell-mass (ms. ent.). 

 The mass fills up almost the entire segmentation cavity (sg. cv.) 

 as is seen in Figs. 34. (PL III.) and 54. (PL IV.). The archen- 

 teron is, at this stage, found sometimes completely obliterated, 

 so that we only see the cell-mass hanging in the segmentation 

 cavity from a pole of the ectoblast sac (PL III., Fig. 34.). In 

 other cases, however, a tolerably spacious archenteric cavity persists 

 within the mes-enteric cell-mass. The nuclei of the entoblast 

 cells undergo some chemical changes, by which they show an 

 increased affinity with nuclear stains, while those of the mesoblast 

 and ectoblast cells retain their original properties (PL IV., Fig. 

 54.). 



