CHAPTEK 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 nut 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 Porifcra the researches of Schulze have clearly 

 demonstrated that both the ova and the spermatozoa take their 

 origin from indifferent cells of the general parenchyma, which may 

 be called mesoblastic. The primitive germinal 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 . 



The following apparent modes of origin of these products have 

 been observed. 



(1) The generative products of both sexes originate in the ecto- 

 derm (epi blast) : Hydra, Cordylophora, Tubularia, all (?) free Gono- 

 phores of Hydromedusge, the Siphonophora, and probably the Cteno- 

 phora. 



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

 genei ative 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 ride Weismann (No. 560). 



