Page:Proceedings of the Royal Society of London Vol 60.djvu/60

52 as described by Schulze, it is obvious th at 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 this takes place while the larva is still in the m aternal 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 a,nus 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. I t is obvious th at 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 ansep a type of development apparently so different as th at of the Sycon am phiblastula.

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 a t 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 quadri'radiates 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 development of the sponge also, I regard this process as one not of blastogenetic, 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.