THE DEVELOPiAIENT OV APLYSIA PUNCTATA. 509 



At this period there are about twelve other endoderm 

 cells, the deinvatives of 4a lying close against the stomo- 

 deeum in the anterior end of the segmentation cavity, those 

 of 4& at the opposite end rather dorsal to the shell-gland, 

 and G and D, with their descendants, also lying at the posterior 

 end against the wall of the shell-gland. The latter are 

 already beginning to form a faii-ly definite row, which will 

 become the posterior well of the stomach. At this stage the 

 greater part of the cavity, which will be the stoniacli, is 

 bounded only by the endomeres A and B, but this soon ceases 

 to be the case. A and B gradually take less and less part in 

 the formation of the wall and give rise to the left liver. 



Thei"e ax-e between fifteen and twenty mesoderm cells in the 

 embryo at this stage. They form an irregular band, which 

 arises at the posterior end near the anal cells, and stretches 

 forwards to the blastopore. The band lies chiefly on the 

 I'ight side, but certain cells are already beginning to pass 

 dorsally and ventrally into the foot. 



We here see that it is possible to speak of a mesoderm 

 band in Aplysia, though it is never clearly defined and soon 

 breaks up. The conditions are very much like those described 

 in Fiona and Umbrella, though the great size of the 

 endomeres in Aplysia has forced the mesoderm chiefly on to 

 the right side. 



There is no secondary mesoderm either in this or later 

 stages. It is present, however, in Fiona. Unfortunately, 

 Heymons woi'ked on Umbrella at a time when the existence 

 of secondary, or ecto-mesoderm, was not recognised, so that 

 its presence or absence in that form is unknown. 



Development of the Organs. 



The embryo rapidly assumes the appearance of the free- 

 swimming larva, and fi-om the beginning of rotation onwards 

 there is but slight alteration in shape and very little increase 

 in size until just before the embryo emerges from the capsule. 



The Yelum. — The velum is, as we have seen, originally a 



