Postlarval Development of Leucosolenia variabilin, //. S p. 51 



rounded by a single layer of dermal cells. Some of the cells of the 

 dermal epithelium then form themselves into groups, usually of three 

 cells, and each cell of such a group secretes the ray of a spicule. The 

 first spicales are usually triradiate, but quite irregular in form, and 

 at their first appearance they are quite superficial, their secreting 

 cells forming part of the general epithelium, but later they become 

 covered by the remaining epithelium, so that the dermal layer 

 becomes divided into an internal connective tissue layer and an 

 external flat epithelium. The process is essentially similar to that 

 occurring in variabilis, except that in the latter the cells of the flat 

 epithelium secrete each a monaxon spicule, which in cerebrum is not 

 the case. 



General Considerations. 



The larva of L. variabilis is of interest as affording a transition 

 from larvae such as that of L. reticulum, to the amphiblastula larva 

 of the Sycons. The larva of reticulum (fig. 7) is composed of (1) 

 ciliated cells, comparable to those of the amphiblastula, of which 

 some (2) at the hinder pole are undergoing modification, and may be 

 compared with the intermediate cells, and of (3) internal granular 

 cells comparable to the posterior granular cells of the amphiblastula. 

 To obtain a larva like that of variabilis from the type represented 

 by reticulum, we must suppose the large cavity of the latter reduced 

 to the extent to which this has occurred in the former. Then the 

 granular cells which are formed at the posterior pole must remain 

 where they are, since the cavity is too small to contain them, and, as 

 more ciliated cells are continually being modified arotmd them, we 

 get a larva with the three kinds of cells arranged as in variabilis. 

 The central cells of variabilis on the origin of which I have no 

 observations to bring forward are probably to be regarded as con- 

 stituting a larval organ, [a, special adaptation of no importance for the 

 postlarval development. 



The development of both reticulum and variabilis points to an early 

 stage in which the larva is composed entirely of similar and equi- 

 valent ciliated cells. I have not seen such a stage in any species, and 

 doubt if it actually occurs in nature ; it is more probable that the 

 process of cell differentiation, begins before the larva is hatched in all 

 cases. In the absence of segmentation stages, it is impossible to 

 decide this question; nevertheless, the facts seem to me to indicate, as 

 the primitive larva in ascon phylogeny, a blastula composed of indif- 

 ferent ciliated cells, in which a second type of cells (the future dermal 

 layer) is formed by modification of certain of the cells. The collar- 

 cell layer of the adult is derived directly from the primitive ciliated 

 cells of the blastula. 



Comparing, now, the larva of variabilis with that of Sycon raphanus, 



