THE DEVELOPMENT OF APLYSIA PUNCTATA. ' 501 



This work was undevtaken to throw light, if possible, on the 

 origin of kidney, heart and pericardium, about which the 

 most diverse statements have been made. It will suffice here 

 to point out that at least six diiferent types of excretory 

 organs have been described in Molluscs, and that the origin 

 and homology of all of them is disputed; while as regards 

 the coelora, opinions differ equally widely. The bearing of 

 our results on these questions will be discussed at the end of 

 this paper. Owing, however, to the fact already mentioned, 

 namely, that it has not yet been found possible to rear the 

 larvae through the metamorphosis, they remain for the present 

 inconclusive. 



For the purpose in view, Aplysia was chosen for two 

 reasons, firstly because it is easy to obtain material at any 

 period of the year, and secondly, because a very careful and 

 complete account of the cell lineage has been given by 

 Carazzi. It was hoped that by beginning at the point where 

 Carazzi left off, it would be possible to follow the develop- 

 ment of the oi'gans, and definitely to ascertain from which 

 cells they arose. To the excellent account of the cell lineage 

 referred to Ave have nothing to add; every cell has been 

 followed up in it to a time when there are more than one 

 hundred, and the history of the endoderm and mesoderm has 

 been traced further. His last description is of an embryo 

 consisting of two hundred and fifty cells, with the velum 

 already developed. 



The development as described by Carazzi may be sum- 

 marised as follows : Segmentation is spii'al, dexiotropic and 

 unequal, the endomeres A and B being far larger than C and 

 D. The great size of these cells makes the cleavage look at 

 first very irregular, but as a matter of fact their destinies 

 show no exceptions to the scheme which Jias come to be 

 recognised as normal in eggs the segmentation of which is of 

 the spiral type. The first three quartettes give rise to all the 

 ectoderm, 4 d entirely to mesoderm. There is no larval meso- 

 derm arising from the ectoderm as has been described in 

 some forms. The endoderm is derived from 3.4, SB, 36' and 



VOL. 55, PART 3. NEW SRRIE.S. 33 



