CELL-LINEAGE. 25 



if 



animals which on the one hand have been very carefully exam- 

 ined as regards their anatomical and general embryological 

 relationships, while on the other hand their cell-lineage has 

 been more exhaustively studied than that of any other forms. 

 These groups are the platodes (more especially the Tiirbellaria), 

 the mollusks, and the annelids. 



That these three groups belong in the same morphological 

 series will probably be admitted by all zoologists, and most 

 will no doubt further agree with the view of Lang, that in the 

 essential features of their organization the platodes are not very 

 far removed from the ancestral type from which the two higher 

 groups have sprung, the former having remained non-metameric 

 like the platodes, while the latter have acquired metamerism. 

 Accepting this view we should expect, if there be any evidence 

 of race-lineage in cell-lineage, to find in the annelids and mol- 

 lusks a common type of cleavage, and one which in its main 

 features may be derived from that of the platode. Recent 

 studies in cell-lineage have, on the whole, justified this expec- 

 tation, and have brought to light some cases of vestigial proc- 

 esses in cleavage which are, I believe, to be reckoned among 

 the most striking and beautiful examples of reminiscence in de- 

 velopment. It is especially to these cases that I wish to direct 

 attention. 



The cleavage of a number of Tiirbellaria and nemerteans, and 

 of many annelids, gasteropods, and lamellibranchs, has now been 

 shown to conform to a common type which, though complex in 

 detail, is exceedingly simple in its essential plan. A few excep- 

 tions there certainly are ; but some of these are apparent only 

 (for example, in the acoelous Tiirbellaria), and are readily re- 

 ducible to the type, while others are undoubtedly correlated 

 with bygone changes in the mode of nutrition of the ovum (as 

 in some of the earthworms and leeches). The most conspicu- 

 ous exception is afforded by the cephalopods, which have a mode 

 of cleavage entirely unrelated to that of the other mollusks ; but 

 the entire development of this group is of a highly modified 

 character. Fully recognizing the real exceptions, we never- 

 theless cannot fail to wonder at the marvellous constancy with 

 which the cleavage of the polyclades, nemertines, annelids, gas- 



