No. 3-] DEVELOPMENT OF MARINE SPONGES. a71 
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, 7.¢. 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 swspect the doctrine that the 
primary germ layers are homologous (I refer of course to the 
general homology maintained by Balfour in his Comp. Emor,, 
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 explanation 
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 
(1) 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 
