o8 ART. 4. N. YATSU I 



now becomes a voluminous sac. Its walls, however, have not as 

 yet differentiated histologically, the mid-gut and the liver-lobes 

 not having appeared. The stomach fills up the greater part of 

 the body-cavity, and when it is fully distended, or when the 

 arm-apparatus is stretched, there is no space left between it and 

 the body walls. 



Coincident with the growth of the stomach and of the body 

 cavity a distinct change takes place in the coelomic wall. Its 

 parietal and visceral layers which have hitherto been composed 

 of large rounded cells, now T assume the characters of a true 

 epithelium. 



The ectoblast of the body wall ventral to the neck remains 

 in the condition of a thin epithelium, and no further differentia- 

 tion occurs at this stage. It is here, however, that in the 5-6 

 p. c. stage the nerve tissue arises. 



IX. STAGE OF 4 PAIRS OF CIRRI. 



As materials of this stage proved very scanty, it is deemed 

 best, not to enter into its more intimate changes. 



The most conspicuous change is the great growth of the 

 shell. The enlargement of the shell (secondary shell) has taken 

 place only along the anterior and lateral margins of the Pro- 

 tegulum and not at all on the hinge-line. This is best seen in 

 the outline drawing of Fig. 76 (PI. V.). At this stage the 

 length of the shell has become somewhat greater than the length 

 of the hinge line. It will be recalled that in the Protegulum 

 these two measurements stood to each other in the ratio of 3 : 5. 



The third and fourth pairs of cirri are formed between the 

 tentacle and the second pair of cirri. The thin central part of 



