No. 3-] DEVELOPME.XT OF MARIXE SPOSGES. 371 



of germ layers and the differentiation of one of the poles, we 

 must now put the question as to the cause of the resembance. 



We have for long been accustomed to regard the two primary 

 germ layers of an embryo as representing the primitive meta- 

 zoan organs, i.e. the outer (nervous) and inner (digestive) layers 

 of a simple two-layered form. It is possible that this view is 

 not an entirely correct one, and that many so-called germ layers 

 are not the ontogenetic representatives of the layers of the 

 metazoan ancestor. And the occurrence of germ layers in an 

 asexually produced embryo may possibly be interpreted as 

 favoring the latter belief. It seems to me, however, that, 

 while it is perhaps permissible to suspect the doctrine that the 

 primary germ layers are homologous (I refer of course to the 

 general homology maintained by Balfour in his Conip. Embr., 

 Vol. 2, p. 286) throughout the metazoa, we are not at present 

 in a position which would warrant our giving up the doctrine. 

 Certain it is that some form of two-layered embryo is found in 

 every group, and that the various forms may be considered as 

 modifications of a type ; and, to my mind, the best e.xplanation 

 of these facts is still the old one, that the germ layers are 

 inheritances from a far distant two-layered ancestor. 



Accepting the premise that germ layers are not independ- 

 ently acquired, but are inheritances from a common stock, we 

 reach the conclusion that an asexually developed embryo 

 (sponge gemmule) can reproduce features of a far distant 

 ancestor (germ layers). 



Coming now to the second point of resemblance (differentia- 

 tion of a pole) between the egg and gemmule larvae of silicious 

 sponges, we have first to ask ourselves, what is the meaning of 

 this curious differentiation of one of the poles in the egg larva 

 itself. This question I am quite unable to answer. Barrois 

 (i) and more recently Keller (10) have regarded the unciliated 

 pole as a blastopore, thus making it possible to compare the 

 larva of silicious sponges with the amphiblastula of calcareous 

 sponges. The basis on which their view rests is, that the 

 endoderm at the pole in question is exposed to the exterior, 

 and this, it is pretty certain, is not the case. The differentia- 

 tion of the pole can have no such deep-seated morphological 



