616 MODE OF EXIT OF GENITAL PRODUCTS. 



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

 etc." JBull. Ac. roy. belpique, Vol. xxxvii. 1874. 



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

 wiss. Zool., Vol. XXXV. 1881. 



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

 Instit. Wiirzlnirg, Vol. i. 1874. 



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

 zool.-zoot. Instit. Wiirzburg, Vol. 11. 1875. 



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

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



Vide also 0. and R. Hertwig {No. 271), KoUiter (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 fea- 

 tures of considerable interest, and displays a fruitful field for future 

 research. 



In the Coelenterata there are no generative ducts. 



In the Hydromedus'de and Siphonophora the generative products 

 are liberated by being dehisced directly into the surrounding me- 

 dium; 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 the 

 methods of transportation of the genital products to the exterior in 

 many of the higher types. 



It has been already pointed out that the body cavity in a very 

 large number of forms is probably derived from parts of a gastro- 

 vascular system like that of the Actinozoa. 



When the part of the gastrovascular system into which the 

 generative products were dehisced became, on giving rise to the body 

 cavity, shut off from the exterior, it would be essential that some mode 

 of transportation outwards of the generative products should be con- 

 stituted. 



In some instances simple pores (probably already existing at the 

 time of the establishment of a closed body cavity) become the gene- 

 rative ducts. Such seems probably to have been the case in the 

 Chsetognatha (Sagitta) and in the primitive Chordata. 



In the latter forms the generative products are sometimes dehisced 

 into the peritoneal cavity, and thence ti-ansported by the ahelominal 

 pores to the exterior (Cyclostomata and some Teleostei, vide p. 514). In 

 Amphioxus they pass by cliehL-icence into the atrial cavity, and thence 

 through the gill slits and by the mouth, or by the abdominal pore (i) to 

 the exterior. The arrangement in Amphioxus and the Teleostei is pro- 

 bably secondary, as possibly also is that in the Cyclostomata ; so that the 

 primitive mode of exit of the generative products in the Chordata is still 

 uncertain. It is highly improbable that the generative ducts of the Tunicata 

 are primitive structures. 



