CHAPTER XXIV. 

 GENERATIVE ORGANS AND GENITAL DUCTS. 



GENERATIVE ORGANS. 



THE structure and growth of the ovum and spermatozoon 

 were given in the first chapter of this work, but their derivation 

 from the germinal layers was not touched on, and it is this 

 subject with which we are here concerned. If there are any 

 structures whose identity throughout the Metazoa is not open 

 to doubt these structures are the ovum and spermatozoon ; 

 and the constancy of their relations to the germinal layers 

 would seem to be a crucial test as to whether the latter have 

 the morphological importance usually attributed to them. 



The very fragmentary state of our knowledge of the origin of 

 the generative cells has however prevented this test being so far 

 very generally applied. 



Porifera. In the Porifera the researches of Schulze have 

 clearly demonstrated that both the ova and the spermatozoa 

 take their origin from indifferent cells of the general paren- 

 chyma, which may be called mesoblastic. The primitive germi- 

 nal cells of the two sexes are not distinguishable ; but a 

 germinal cell by enlarging and becoming spherical gives rise 

 to an ovum ; and by subdivision forms a sperm-morula, from 

 the constituent cells of which the spermatozoa are directly 

 developed. 



Ccelenterata. The greatest confusion prevails as to the 

 germinal layer from which the male and female products are 

 derived in the Ccelenterata 1 . 



1 E. van Beneden (No. 556) was the first to discover a different origin for the 

 generative products of the two sexes in Hydractinia, and his observations have led to 

 numerous subsequent researches on the subject. For a summary of the observations 

 on the Hydroids vide Weismann (No. 560). 



