34 



development may go no further than the production of a 

 hollow diverticulum of the body-wall of the parent known 

 as a sporosac or sporophore. 



It is worthy of notice that the free medusa in the course 

 of its development passes through in succession the stages 

 of sporosac, disguised medusa, and attached medusa; so that 

 these latter may be regarded as due to arrested development 

 of the medusa at an earlier or later stage. That this view 

 is correct rather than one which would regard the sporosac? 

 disguised medusa, and attached medusa as representing 

 stages in the gradual progressive evolution of the free me- 

 dusa, is evident from the consideration that the disguised 

 medusa and attached medusa, which have all the parts of 

 the free medusa fitting it for independent existence but 

 never have an opportunity of employing them, could never 

 have arisen by a process of natural selection from the spo- 

 rosac, for the possession of a swimming bell that is never 

 opened could clearly be of no advantage. 



Hence the forms with free-swimming medusae must be 

 regarded as the most primitive, and those with attached or 

 disguised medusae, or with sporosacs, must be viewed as 

 derived from these by abortion, more or less complete, of 

 the various parts of the free medusa, such abortion being 

 intimately associated with the early or premature ripening 

 of the sexual products. 



Weismann, in the work alluded to above, has shown that 

 the genital cells may arise in parts other than those in 

 which they are ultimately lodged, and indeed before the 

 appearance of these latter, into which they migrate later on. 

 In some cases this may be carried so far that the genital 

 cells arise in the body-wall of the primary zooid not only 

 before the commencement of the development of the gono- 

 phore, or sexual bud, but even before the first trace of the 

 appearance of the branch on which the gonophore will be 

 borne. A good example of this is afforded by the fresh- 



