52 On the Larva and Development of Leucosolenia variabilis. 



as described by Schulze, it is obvious that the development is essen 

 tially similar in both, the chief difference being with regard to the 

 periods at which the various events take place. In both the granular 

 cells increase greatly in number, but in raphanus this takes place 

 while the larva is still in the maternal tissues, ' as is obvious from 

 Schulze's figures,* and the larva is hatched in a condition similar 

 to that of variabilis when about to fix. In variabilis the granular 

 cells do not surround the ciliated cells until after fixation ; in raph- 

 anus this process is begun while the larva is still swimming, and 

 the granular cells may even give rise to spicules (monaxons) during 

 the free swimming period (Metschnikoff, loc. cit.). It is obvious 

 that in Sycon we have before us a hastening and shortening of the 

 development, and, allowing for these embryological adaptations, we 

 are able to understand how, from a larva such as that of reticulum, 

 there has arisen a type of development apparently so different as 

 that of the Sycon amphiblastula. 



The most important event in the post-larval development is the 

 differentiation of the dermal layer into the outer epithelium and the 

 inner connective tissue layer. This might seem at first sight to be a 

 process comparable to the formation of a new layer, a mesoderm ; so 

 that from this period onwards the sponge would be a three-layered 

 organism. I do not, however, take this view, for the following reason. 

 The immigration of cells from the epithelium to form the layer of 

 triradiates is not an event, like the formation of a germ layer, which 

 takes places once and for all in the life cycle of an individual, but it 

 goes on whenever new triradiates are formed. In adult ascons I have 

 found that the triradiates and the basal rays of the quadriradiates 

 arise from cells of the outer epithelium which migrate inwards and 

 arrange themselves into groups to form spicules, each ray being 

 secreted by one cell or by cells derived from the division of a 

 single cell. In the adult also the nuclei of the spicule secreting cells 

 diminish in size after quitting the epithelium. Hence in the develop- 

 ment of the sponge also, I regard this process as one not of blasto- 

 genetic, but of histogenetic significance. The fact that in variabilis 

 the epithelial cells also secrete spicules is to my mind a decisive 

 proof of the unity of the dermal layer.f 



' Zeitschr.f. Wiss. Zool.,' vol. 25, suppl., Taf. XX and fig. 3, Taf. XIX. Schulze 

 refers this increase in the number of the granular cells to their multiplication by 

 cell-division, but as the granular cells do not at the same time decrease in size, it 

 seems more probable that their increase is due, as in variabilis^ to their numbers 

 being recruited from the clear (ciliated) cells. 



f Schulze has also figured very clearly the relation of the dermal cells to the 

 monaxon spicules, one epicule to each cell, in the young fixed stages of Sycon 

 raphanus (' Zeitschr. f . Wiss. Zool.,' vol. 31, pi. XIX, figs. 10, 11), although he 

 states in the text that the spicules arise in the hyaline substance between the two 

 layers. 



