82 GERM-CELL CYCLE IN ANIMALS 



are therefore also ectodermal. Van Beneden (1874), 

 from investigations on Hydractinia, Clava, and Cam- 

 PANULARiD^, confirms his earlier results and again 

 maintains that the ova arise in the entoderm. The 

 brothers Hertwig (1878) decided that the germ cells 

 of Hydromedus^ arise from the ectoderm and those 

 of the ScYPHOMEDUs^ and Anthozoa from the 

 entoderm. In a second paper, Kleinenberg (1881) 

 reports the ova of Eudendrium as of ectodermal 

 origin. Varenne (1882) maintains that both the 

 ova and the spermatozoa of half a dozen species 

 examined arise from entoderm cells of the young 

 blastostyle before the appearance of medusa buds. 

 The results of Weismann's extended studies were 

 published in a monograph (1883), and later (1884) a 

 brief general account appeared. 



From this time until the present day almost every 

 year has witnessed one or more contributions to the 

 subject of the origin of the germ cells in coelenterates, 

 and a perusal of this mass of literature shows that 

 the problem is not yet solved. 



Hydra. The fresh- water polyp. Hydra, has been 

 employed for germ-cell investigations more often 

 than any other coelenterate, and a number of de- 

 tailed papers have appeared within the past ten 

 years upon this genus. Among the earlier workers 

 who actually saw the egg should be mentioned 

 Trembley (1744), Rosel V. Rosenhoff (1755), Ehren- 

 berg (1836) and Leydig (1848). The processes 

 involved in oogeneses were not clearly determined, 

 however, until Kleinenberg's classic investigations 



