22 ART. 4. — N. YATSU : 



arm-apparatus and the lateral growth of the mantle lobes, as is 

 seen in Fig. 42. (PI. III.) which represents an embryo at this 

 stage seen from the ventral and slightly from the anterior side. 

 The arm-apparatus {am. ap.) comes to assume the form of a disc, 

 almost circular or elliplical in outline. Moreover from this stage 

 onward the arm-apparatus is not situated just midway between 

 the dorsal and ventral lobes of the mantle but approaches nearer 

 the former than the latter, so that it appears to project from 

 the dorsal lobe. The anterior free portion of the dorsal lobe is, 

 therefore, shorter than that of the ventral lobe. This relation of 

 the mantle and arm- apparatus will be referred to later. The 

 cells composing the arm-apparatus have become gradually smaller, 

 and consequently its warty appearance has vanished. On the 

 ventral median plane of the arm-apparatus the mouth (m.) is 

 seen. The mantle now comes to acquire a nearly semi-circular 

 form, having the posterior straight edge corresponding to the 

 diameter. The mantle increases greatly in size, especially at the 

 anterior margin so as to nearly hide the arm-apparatus as is seen 

 in an embryo of a similar stage drawn in a side-view (PI. III., 

 Fig. 43.). In another embryo the segmentation cavity was seen 

 just inside the posterior body wall. 



Two embryos a little further advanced are shown in Fig. 44. 

 (PI. III.) in the ventral view, and in Fig. 45. (PL III.) in the 

 dorsal view. In these we percieve the following changes : the 

 rupture of the vitelline membrane, the formation of the cirri and 

 the tentacle, and the secretion of the shell. The period in which 

 the bursting of the vitelline membrane takes place varies con- 

 siderably ; in some cases it takes place at this stage, while in 

 others it is delayed still later. The vitelline membrane bursts on 

 the anterior end of the embryo ; this is probably caused by the 



