THE EMBRYOLOGY OF THE PORIFERA. 229 



layer of columnar flagellated cells, those at the posterior 

 end being broader, more granular, and with larger nuclei 

 than those anteriorly. After a short time one pole of the 

 larva is invaginated into the other, but in this process ab- 

 normalities are frequent, and it is not certain which method 

 is normal ; H eider 1 believes that normally the posterior pole 

 is invaginated into the anterior. The larva fixes by the 

 orifice of invagination, and the invaginated cells become 

 the collar cells. 



Delage's explanation of this development is that the 

 cells of the blastula constitute an indifferent layer, con- 

 taining potentially both endoderm and ectoderm, which 

 only become differentiated after fixation. 



Maas, on the other hand, regards the blastula, so 

 called, of Oscarella as really equivalent to the amphi- 

 blastula of Sycon, the hinder granular cells of the former 

 being the equivalent of the granular cells of the latter, 

 with the difference that the granular cells of Oscarella 

 are ciliated, as in the adult ectoderm. He thinks that 

 normally the anterior pole of the larva is invaginated 

 into the posterior, a process strictly comparable to the 

 invagination of the Sycon larva or the overgrowth of the 

 flagellated cells in siliceous sponge larvae. 2 



Maas's view seems the most probable, for an additional 

 reason which both he and Delage have overlooked. In 

 1884 Sollas 3 published a memoir on the development of 

 Oscarella, in which he arrived at the conclusion that the 

 embryo developed into a young sponge before leaving the 

 maternal tissues, a conclusion which was justly set aside 

 and refuted by H eider. What Sollas really did show, 

 however, was that the embryo undergoes a distinct inva- 

 gination before leaving the mother sponge, an invagination 



1 Zur Metamorphose der Oscarella lobularis. Arbeiten d. Zool. Inst. 

 Wien, bd. vi., 1886. 



2 1 expressed exactly the same opinion as that put forth here in my 

 lectures in 1892, without, however, having discussed the matter at all 

 with Dr. Maas. 



s Quart. Journ. Micr. Science, N.S., xxiv., 603-621, pi. xxxvii. 



