450 EIOHAED EVANS. 



idea that the sponge larva must be subjected to the dogmas 

 of the '^ germ-layer theory," that, having discovered both 

 "endoderm" and "mesoderm" in the inner mass, he could not 

 conceive the canals and chambers as developing in situ, which 

 is their true origin. In many cases the short canals which 

 have been described in the present account do not seem to 

 communicate with the larval cavity at all, save in exceptional 

 cases, and for that reason cannot be produced as evaginations 

 from it. 



The swollen or expanded ends described by Maas are really 

 the cell groups of the present memoir, and become the flagel- 

 lated chambers. They are derived, as has been shown in 

 Appendix B, from the cells with vesicular nuclei, by fragmen- 

 tation of the nuclei and susequent division of the cell body. 

 The canals which he described as communicating with the 

 chambers on the one hand, and with the larval cavity on 

 the other, have been shown to have their lining formed by 

 cells with granular nuclei developed in situ. They con- 

 sequently belong to the same class as those which line the 

 larval cavity, as stated by Maas, though for erroneous reasons. 

 In the next place we have to examine Gotte's view, namely, 

 that the cells of the inner mass {'' endoderm ") give rise 

 to the whole sponge, the "ectoderm" being completely 

 lost. The flagellated chambers are described by him as being 

 produced by groups of cells — each of which has arisen from 

 a single cell — enclosing a cavity, just as has been described at 

 full length in the present account. Gotte did not, however, 

 distinguish clearly between yolk bodies and fragmenting nuclei. 

 A somewhat similar view of the origin of the flagellated cham- 

 bers was put forward by Saville Kent (6), though he did not 

 understand the true nature of sponges, and held that they were 

 Protozoa. Dendy (2) also described a similar origin for the 

 flagellated chambers in some horny sponges (Stelospongus). 



With regai'd to the development of the lining of the canals 

 from cells of the inner mass 1 agree with Gotte. But for 

 several reasons 1 hold that the cells which line the canals 

 belong to quite a diff'erent class from those which become the 



