THE EMBRYOLOGY OF THE PORIFERA. 227 



rounding an inner mass of coarsely granular cells with 

 large nuclei. It seems almost impossible to believe that 

 the flagellated cells in the larva of Ascetta do not, as in 

 the other type, become converted into the collar cells of 

 the adult. Such a reversed development would involve 

 grave morphological difficulties, and there are in addition 

 histological reasons against its occurring. In the adult 

 Ascetta, as in other sponges, the nuclei of the collar cells 

 are the smallest in the sponge, their diameter being in fact 

 scarcely half that of the ectoderm cells. In the flagellated 

 cells both of the Ascetta larva 1 and of the larva of Corna- 

 cuspongice the nuclei are similarly very much smaller than 

 those of the inner mass. 



What in all probability really takes place in the develop- 

 ment of Ascetta is that the larva fixes by the anterior pole, 

 the inner mass bursts out at the posterior (upper) surface, 

 and grows round the flagellated cells, which thus come to 

 lie internally, just as they do in Sycon or Esperia? This 

 supposition becomes still more probable if MetschnikofT's 

 figures be carefully studied in the light of this theory, 

 especially figs. 13, 14, 15 of his tafel xxiii. 3 These three 

 figures can easily be interpreted, as Maas has pointed out, 

 as representing the bursting out of the inner mass (fig. 

 14) and its growth round the flagellated cells (fig. 15). 



1 Compare especially the figures of Schmidt, toe. at., taf. xv. ; figs. 5, 

 6 and 7. 



- The above conclusions were arrived at by me, as Maas has kindly 

 stated [(2), p. 419, footnote], when I was giving a course of lectures on 

 sponges at Oxford in 1892. At that time only Maas's work on Esperia 

 (1) and the two preliminary accounts of Delage had appeared (Comptes 

 Rendus, ex., p. 654, and cxiii., pp. 267-269). I wrote to Dr. Maas and 

 asked him his opinion on the subject, and in his answer to my letter he 

 expressed the same opinion as I had already arrived at, and which he 

 has since published. Just after this Delage's work (1) appeared, stating 

 the same opinion again. M. Delage of course has the priority in the 

 matter, but it is interesting that three naturalists in three different 

 countries should have simultaneously and independently formed the 

 same views upon this subject. It will be still more interesting when 

 some investigator describes how Ascetta really does develop. 



s Loc. at. 



