82 KICHARO EVANS. 



the gemmule ; but when I saw that the material at my disposal 

 contained gemmules in all stages of development I thought 

 it would be a mistake not to describe it. Further, I was en- 

 couraged to do so by Professor Weldon, to whom I am greatly 

 indebted both for the free use of his laboratory and all its 

 resources, and for much invaluable assistance, especially in 

 connection with the literature on the subject. I shall first 

 o-ive a summary of what is already known of the gemmule. 

 I shall then proceed to describe my own observations, the 

 method followed being that of tracing the origin and subse- 

 quent changes of the various cells which take part in the 

 process, this method being considered simpler and more intel- 

 ligible than that of giving a complete description of the differ- 

 ent stages of development. The reader can easily make out 

 for himself, by examining the figures 8, 9, .... 17, the true 

 relation of the changes in the different parts of the developing 

 gemmule much better than by reading the best possible 

 description. Finally, I shall review previous accounts and 

 compare my own conclusions with them. 



II. HiSTOEICAL EeVIEW. 



Carter (2), who was the first to attempt an explanation of 

 the origin of the gemmule, which he terms the seed-like body, 

 writes as follows : — " At the earliest period of development in 

 which I have recognised the seed-like body it has been com- 

 posed of a number of cells, united together in a globular or 

 ovoid mass (according to the species) by an intercellular 

 substance. In this stage, apparently without any capsule, 

 and about half the size of the full-developed seed-like body, 

 it seems to lie in a cavity formed by a condensation of the 

 common structure of the sponge immediately surrounding it. 

 It passes from the state just mentioned into a more circum- 

 scribed form, then becomes surrounded by a soft, white, 

 compressible capsule ; and finally thickens, turns yellow, and 

 develops upon its exterior a firm crust of siliceous spicules." 

 He says with regard to the origin of the genimule, "I do not 



