7 6 SPONGES 



non-ciliated dermal cells at the posterior pole must remain where 

 they are, and do not immigrate into the interior. As the process 

 of cell modification continues, there is a constantly increasing 

 accumulation of rounded non-ciliated cells at the posterior pole. 

 The result is a larva with two sharply differentiated regions, an 

 anterior ciliated, and a posterior non-ciliated pole. Just such 

 a larva is found in Leucosolenia, in which, when newly hatched 

 (Fig. 59, 2), the non-ciliated region is absent or comparatively small, 

 hut increases continually at the expense of the ciliated region. 

 Between the two regions is an equatorial zone of cells intermediate 

 in their characters, and in process of modification, and the centre of 

 the larva is occupied by the archaeocytes or central cells. The larva 

 swims about until it is about equally composed of ciliated gastral 

 cells, and non-ciliated dermal cells (Fig. 59, 3). It then fixes by the 

 anterior pole, and the ciliated cells are overgrown by the amoeboid 

 dermal cells. In other respects the development is essentially 

 similar to that of Clathrina. 



From the larva of Leucosolenia it is but a slight step to the well- 

 known, but often misunderstood, development of Si/con. In this form 

 the ovum undergoes a total and regular segmentation (Fig. 60, a, b, c) 

 and produces a blastula, in which certain cells at one spot, the future 

 hinder pole, are marked out by their larger size, and darker granular 

 appearance (Fig. 60, d) ; these are the archaeocytes, comparable to the 

 posterior granular cells in Clathrina. 1 The clearer cells (histocytes) 

 become columnar, and acquire flagella, while the granular archaeo- 

 cytes pass into the interior of the segmentation cavity, which they 

 nearly fill, and are completely enclosed by the clearer cells ; this is 

 the so-called pseud ogastrula stage (Fig. 59, 2). The cells at the 

 hinder pole next begin to become modified in the usual way into 

 rounded non-ciliated cells, comparable in every way to those of the 

 inner mass of Clathrina, and the number of non-ciliated cells, at 

 first small, increases continually at the expense of the ciliated cells, 

 until the two kinds contribute to the composition of the embryo in 

 about equal proportions. At this stage, when the blastogenesis is 

 complete, the larva is hatched and swims freely ; it is made up of 

 columnar flagellated cells at the anterior pole, rounded, non-flagel- 

 lated cells at the posterior pole, and a central mass of granular 

 amoebocytes (Fig. 59, 3, and Fig. 60, e). During the free swimming 

 period the ciliated gastral layer becomes partially overgrown by the 



1 The account here given differs from that of Schulze, who regarded these granular 

 cells as the future dermal layer ; for this reason Schulze distinguished the posterior 

 non-ciliated cells of the amphiblastula as granular cells (Kornerzelleri), from the 

 flagellated cells, though as a matter of fact the latter are in reality the more granular 

 of the two, since they contain yolk, which in the dermal cells becomes worked up 

 and absorbed more quickly. The statements here made are based upon my own ob- 

 servations upon Leucosolenia, and the figures of Barrois for Sycon and Grantia ; see 

 also Dendy (1889). 



