THE DEVELOPMENT OF APLYSIA PUNCTATA.. 515 



there are never less than four present, it is not easy to 

 explain. 



The Primitive Kidneys. — These organs, described by 

 Trinchese as nephrocysts, consist each of a large, much- 

 vacuolated cell with a small nucleus, lying one on either side 

 of the body at the base of the vehim. In the living embryos 

 they are very obvious on account of the brightly-coloured oily 

 globules which they contain (PI. 22, Kl, in figs. 4 to 9). 

 They appear to be characteristic of Opisthobranch larvte, and 

 presumably constitute temporary excretory organs. Mazza- 

 relli ascribes to these cells a mesodermal origin, and when 

 first seen they certainly appear to lie well inside the body in 

 both Fiona, according to Casteel, and Aplysia, All previous 

 attempts to trace them back to their origin in segmentation 

 have failed, and in spite of employing various methods of 

 preserving and staining we have been equally unsuccessful. 

 We consider it probable, however, that they are of ectodermal 

 origin, of the same nature as the anal cells, which sink below 

 the surface and lose their excretory function at a time when 

 the secondary kidney is developed to take it on. 



The Alimentary Canal. — We left the segmentation 

 cavity at a stage when it was largely bounded by the two 

 large endomeres, A and B. PI, 22, fig. 3 represents a slightly 

 later stage. The number of endomeres has increased, not by 

 the division of the large blastomeres, A and B, but of: the 

 smaller endoderm cells. In the anterior region of the seg- 

 mentation cavity the two large endomeres do not, therefore, 

 contribute to form its boundary to such an extent as before. 

 The cells which give rise to the wall of the cavity iu this 

 region are chiefly derived from 4ft and D. The posterior 

 wall of the cavity is still more complete, and its constituent 

 cells are the derivations of 41, C, and Z). This corresponds 

 very closely with the condition of things in Fiona, where the 

 posterior Avail is formed by oB, bh, 4c, hC, 5c, 4D, and 

 oA. Umbrella agrees very nearly in this respect with Aplysia 

 and Fiona. 



At a slightly later stage (PI. 22, fig. 4) the stomodasum 



