THE EMBRYOLOGY OF THE PORIFERA. 225 



rarely anteriorly as in Aplysilla. The larva fixes by the 

 anterior pole, and the granular cells come to surround the 

 flagellated cells. The latter give rise to the collar cells of 

 the adult, while the granular cells form (1) all the flattened 

 epithelium, and (2) all the mesoderm, so called, of the adult 

 sponge. 



The resemblance of this type of development to that 

 occurring in Sycon, with its amphiblastula larva, is obvious. 

 The flagellated cells of the amphiblastula correspond in 

 origin, position, and destination to those of siliceous 

 larvae. The granular cells, in like manner, of the amphi- 

 blastula, are homologous with the inner mass of the siliceous 

 larvae. Delage admits that the cellules intermediaires 

 and amceboides of his larvae, while wanting in the amphi- 

 blastula of Sycon, "are yet potentially contained in the 

 granular cells," which he regards as equivalent to the 

 epidermiques. "The larva," he says, "of Sycandra 

 can be considered as a larva of Esperia or Reniera 

 reduced to the cells of its external envelope," with the 

 epidermic cells confined to the naked pole. This homology 

 between the granular cells of Sycon larvae, and the whole 

 inner mass of the siliceous sponge larva, is, as Maas points 

 out, an additional argument for regarding the latter as repre- 

 senting a single layer. In the amphiblastula, the flagellated 

 layer is less strongly developed, and does not enclose the 

 granular cell mass. Not only, however, do variations in 

 this respect occur in siliceous larvae, but the existence of 

 a so-called pseudogastrula stage has long been known in 

 Sycons, in which, shortly before the larva leaves the mater- 

 nal tissues, the granular cells are almost entirely surrounded 

 by the flagellated cells. The segmentation of the ovum, 

 the embryonic development of the larva, and the metamor- 

 phosis are all very similar in the two larvae. Delage 

 points out that the invagination of Sycon, the so-called 

 amphigastrula, and the penetration of the flagellated cells 

 into the interior in siliceous larvae are "two phenomena 

 ... of fundamentally the same order," but this is even 

 more the case if we accept Maas's account of the metamor- 

 phosis. 



16 



