748 MODE OF EXIT OF GENITAL PRODUCTS. 



In Mammalia the evidence of the origin of the spermato- 

 spores from the germinal epithelium is not quite complete, but 

 there can be but little doubt of its occurrence 1 . 



In Amphioxus Langerhans has shewn that the ova and 

 spermatozoa are derived from similar germinal cells, which may 

 be compared to the germinal epithelium of the Vertebrata. 

 These cells are however segmentally arranged as separate 

 masses (vide Vol. II. p. 54)- 



BIBLIOGRAPHY. 



(554) G. Balbiani. Lemons s. la generation des Vertebra. Paris, 1879. 



(555) F. M. Balfour. "On the structure and development of the Vertebrate 

 ovary." Quart. J. of Micr. Science, Vol. XVIII. 



(556) E. van Beneden. "De la distinction originelle du tecticule et del'ovaire, 

 etc." Bull. Ac. roy. belgique, Vol. xxxvn. 1874. 



(557) N. Kleinenberg. "Ueb. d. Entstehung d. Eier b. Eudendrium." Zeit. 

 f. wiss. Zool.) Vol. xxxv. 1881. 



(558) H. Ludwig. "Ueb. d. Eibildung im Theirreiche." Arbeit, a. d. zool.- 

 zoot. Instil. Wiirzburg, Vol. I. 1874. 



(559) C. Semper. "Das Urogenitalsystem d. Plagiostomen, etc." Arbeit, a. 

 d. zool.-zoot. Ins tit. Wiirzburg, Vol. n. 1875. 



(560) A. Weismann. "Zur Frage nach dem Ursprung d. Geschlechtszellen bei 

 den Hydroiden." Zool. Anzeiger, No. 55, 1880. 



Vide also O. and R. Hertwig (No. 271), Kolliker (No. 298), etc. 



GENITAL DUCTS. 



The development and evolution of the generative ducts is as 

 yet very incompletely worked out, but even in the light of our 

 present knowledge a comparative review of this subject brings to 

 light features of considerable interest, and displays a fruitful 

 field for future research. 



In the Ccelenterata there are no generative ducts. 



In the Hydromedusae and Siphonophora the generative 

 products are liberated by being dehisced directly into the 

 surrounding medium ; while in the Acraspeda, the Actinozoa 

 and the Ctenophora, they are dehisced into parts of the gastro- 

 vascular system, and carried to the exterior through the mouth. 



The arrangement in the latter forms indicates the origin of 



1 An entirely different view of the origin of the sperm cells has been adopted by 

 Balbiani, for which the reader is referred to his Memoir (No. 554). 



