28 ART. 4. — N. YATSÜ : 



rear from the eggs, the tips of the cirri were tinted a very light 

 yellowish-brown. 



ß. Internal structure. 



1. Alimentary Canal. 



Fig. 54 (PI. IV.) is an embryo of nearly the same stage as 

 the one represented in Fig. 34 (PI. III.), cut obliquely through 

 the blastopore. In this section we see that the entoblast is 

 arranged epithelially and constitutes the inner part of the mes- 

 enteric cell mass. The entoblast cells already show a stronger 

 affinity for stains than the mesoblast. As the embryo grows the 

 blastopore is closed, the entoblast forming a sac (PI. IV.). The 

 stomodreum is next formed at the spot where the blastopore was 

 present, and consists of the cells which in their behavior toward 

 stains are similar to those of the arm-apparatus. 



The entoblastic sac, just mentioned, elongates and is differen- 

 tiated into two parts : the anterior half becomes the posterior 

 part of the oesophagus, retaining its great thickness, while the 

 posterior part becomes stomach, its walls becoming thin. Fig. 62 

 is a frontal section of a slightly advanced embryo. In this there 

 is seen the posterior half of the oesophagus, as also the stomach 

 highly compressed and appearing as if it were giving off a pair 

 of diverticula. At first sight this appearance may give the im- 

 pression that the Anlagen of the coelom are being formed after 

 the enterocoelic type. But a closer examination reveals that it is 

 nothing but the collapsed stomach (PI. IV., Figs. 62, 65.). Such 

 an appearance is not found in the embryos fixed in a stretched 

 condition. 



