THE DEVELOPMENT OF APLYSIA PUNCTATA.. 507 



We propose first to describe in some detail the earliest 

 stage Avhich we have investigated, and then to follow the 

 development of the various organs separately up to the time 

 when the larva becomes free swimming. 



Structuke op the Embryo at the End oe Segmentation. 



PI. 22, fig. 1 is an external view of an embryo shortly before 

 rotation begins, and fig. 2 in the same plate shows the cells 

 which have sunk below the surface at the same stage. The 

 total number of cells is more than 300. The ectoderm forms 

 a tiiin and uniform layer covering the surface of the embryo. 

 Round the anterior end there is a ring of somewhat larger 

 cells derived from the B quartette, which bear long cilia and 

 form the velum. At the opposite pole are the two anal cells 

 2(^22221 j^jj(j 2(?22222^ whlcli are very prominent and project 

 markedly from the surface, thus forming a convenient means 

 of orientating the embryo with certainty. Near their bases, 

 are small nuclei which are sometimes difficult to see, and were 

 not noticed by Blochmann (2). Their cytoplasm is very much 

 vacuolated. They presumably function as temporary excre- 

 tory organs. They are characteristic of Opisthobrauch 

 larvee, though in some cases, as iu Fiona, described by Casteel 

 (6), they are small and but little differentiated from the 

 other ectoderm cells. It is well known that in certain other 

 Gastropod larvae ectoderm cells of considerable size are found 

 projecting from the surface ; Glaser (8) has described them 

 in Fasciolaria, where they occur singly or in groups of two 

 or three together. These would appear to be comparable 

 to the anal cells of Aplysia, but iu Fasciolaria their position 

 is variable, being almost anywhere on the surface. In the 

 region of the A quartette there is a slight projection, the 

 cells being somewhat enlarged. This is the rudiment of the 

 foot. Between the foot and velum is the blastopore on the 

 ventral surface, and round it the ectoderm cells are beginning 

 to sink in. Though the blastopore is at this time very 

 small, we have always found it perfectly distinct, and in 

 this we agree with Carazzi in contradiction to Mazzarelli and 



