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 HypRomEpvus2 arise from the ectoderm and those 
of the ScypHomEepus® and ANntTHOozoA 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 ccelenterates, 
and a perusal of this mass of literature shows that 
the problem is not yet solved. 
Hypra. The fresh-water polyp, Hydra, has been 
employed for germ-cell investigations more often 
than any other ccelenterate, 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), Résel V. Rosenhoff (1755), Ehren- 
berg (1836) and Leydig (1848). The processes 
involved in odgeneses were not clearly determined, 
however, until Kleinenberg’s classic investigations 
