444 RICHARD EVANS. 



In the following discussion of previous works we shall take 

 first each class of cells in the larva separately, and consider 

 the various theories that have been held with regard to it. 

 This method of treatment will result in less repetition than 

 that of dealing with the various accounts in the order in which 

 they were published. We may consider, in the first place, 

 the fate of the flagellated layer ; and, in the second place, the 

 dififereutiation of the inner mass, both during the free-swimming 

 period and the pupal stage. 



A. The Fate of the Flagellated Layer. 



The views held as to the fate of the cells which constitute 

 the layer in question may be divided at the outset into two 

 classes: first, that of Gotte (5), who holds that they are cast 

 off during the metamorphosis and lost altogether ; secondly, the 

 views of all other authors, who hold that they become of use to 

 the young sponge in some form or other. 



This second class of views may be divided into three sub- 

 classes : first, the view of Ganin (4) and Maas (7), that they 

 become flattened out to form the cells of the flat epithelium ; 

 secondly, the view of Delage (1) and Maas (8), who hold that 

 they become the collar-cells of the young sponge, though tiiese 

 two authors difi'er considerably as to the details of the process ; 

 and thirdly, the novel view of Noldeke (13), who holds that 

 they are devoured by the cells with vesicular nuclei. 



In considering the view held by Gotte (5), it must be 

 admitted that larvae such as that represented in section in 

 figs. 29 and 29 a could possibly dispense with the flagellated 

 layer, for the inner mass already contains all the elements 

 necessary for the building up of the young sponge. But I have 

 never seen any signs of these cells being thrown off". The 

 only explanation of what seems to be an error on the part of 

 Gotte is that he observed specimens which had in some way or 

 other become injured. However, the credit belongs to him of 

 discovering that the 'flagellated cells do not become the flat 

 epithelium or " ectoderm." 



