THE MORPHOLOGY OP THE SEXUAL ORGANS OP HYDRA. 325 



a more or less immature condition. In some instances the gonophore 

 is a fully-formed medusa, which, however, never detaches itself from 

 the colony, such a gonophore being called an attached medusa ; in 

 other cases development stops at a still earlier stage, giving rise to a 

 disguised medusa, in which all the essential parts of the medusa are 

 present, but in an unexpanded condition ; and, finally, 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, within 

 the walls of which the ova or spermatozoa are matured. 



It is worthy of notice that the free medusa in the course of its de- 

 velopment passes through in succession the stages of sporosac, dis- 

 guised 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 repre- 

 senting stages in the gradual progressive evolution of the free medusa, 

 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 em- 

 ploying them, could never have arisen by a process of natural selection 

 from the sporosac, for the possession of a swimming bell that is never 

 opened could clearly be of no advantage. 



Hence the forms with free-swimming medusse must be regarded as 

 the most primitive, and those with attached or disguised medusse, 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 of Hydrozoa 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 subsequently be borne. A 

 good example of this is afforded by the fresh-water genus Cordylo- 

 phora, in which the ova arise in what Weismann calls the germinal 

 zone of the primary zooid, then migrate into the lateral branch of the 



