20 ART. 4. N, YATSU I 



is a ring-like area (mt. fd.) which I shall designate as the 

 mantle fold, as it soon becomes the mantle. Between the arm- 

 ridge and the mantle fold there is a shallow furrow. On what 

 becomes the sides of the body there soon appears on each side, 

 at the place marked with stars in the figure, a shallow depression 

 on the mantle fold. By these two depressions we can now 

 distinguish in the embryo the two sides. Fig. 36. (PI. III.) 

 represents the same embryo viewed from either the dorsal or 

 ventral side. In this figure we see more clearly the relation of 

 the arm-ridge, the mantle fold, and the shallow furrow between 

 them. Moreover we see that the posterior face of the embryo 

 fchows a gentle vaulting. 



An embryo a little more advanced than that just described 

 is represented in Fig. 37. (PI. III.) in the same view as in Fig. 

 35. (PI. III.). At this stage an invagination begins to be marked 

 at the future ventral median part of the arm-ridge. Proceeding 

 dorsally and posteriorly it forms the stomodseuni (stmd.). Inter- 

 rupted on the ventral side by this invagination the arm-ridge 

 comes to assume the form of the letter U whose curved portion 

 is directed dorsally. Both the arms of the U are symmetrical. 1 

 The lateral depressions on the mantle fold ( * * ) become deeper 

 than before, and are ready to cut the mantle fold into two lobes. 



Next the stomodœum comes into communication with the 

 archenteron. The former constitutes the anterior half of the 

 oesophagus. Fig. 38. (PL III.) is an embryo of this stage, 

 pressed by the cover glass from the antero-ventral direction and 

 drawn on a much smaller scale than Fig. 37. The arm-ridge is 

 pressed down dorsally and the depressions on the mantle fold 



1. In the embryo drawn in the figure the symmetry is a little disturbed. This must 

 not be taken as a normal case. 



