The Sexual Phases of Myzostoma. 
271 
not float about freely during their growth, but are stowed away 
in masses in the diyerticula is a secondary condition easily ac- 
counted for. 
There are greater but by no means insuperable obstacles in the 
way of a comparison of the testes of Myzostomes with those of 
Chaetopods. In the youngest stages of M. glabrum examined, the 
spermatogonia could be traced to the cells of the peritoneum. During 
their proliferation these spermatogonia do not project into the body- 
cavity like the oogonia of many species, but into the parenchyma, 
which soon encloses them in masses, very much as the ovarian 
Stroma encloses the Pflüger’s columns of the Vertebrate, thus cut- 
tiug them off from the peritoneum. Notwithstandiug this Separation 
from their place of origin, the male reproductive elements, when 
they become mature, are in many species set free into the body- 
cavity, whence they may leave the body through the female 
genital opening. Even in forms like M. glabrum and cirriferum^ 
which have vasa deferentia and special male openings through the 
penes, many spermatozoa find their way into the body-cavity and 
escape with the ova through the orifice of the »uterusa. 
In forms like M. belli and probably also M. cryptopodium^ where 
the vasa deferentia and penes have been lost, all the spermatozoa 
must pass out through the body-cavity like the ova. In these cases 
the typical Chaetopod condition has probably been reacquired se- 
condarily. 
2. The structure of the ovaries. In their minute structure 
the ovaries of Myzostoma are readily compared with those of other 
ChaetQpoda. In both groups two kinds of cells are early differentiated 
— the reproductive cells proper (oogonia and oocytes) and what I 
have called the accessory cells. The latter are nothing more 
nor less than the »Nährzellen« of other authors! 
A very striking similarity to the condition seen in Myzostoma is 
exhibjted .. by the Polychaete Ophryotrocha puerilis , as described 
simultaneously by Braem ('93) and Korschelt ('93). The descriptions 
of both authors agree except in a few minor details. Of the two 
kinds of cells which may be distinguished in the ovaries of Opliryo- 
trocha^ the oocyte is larger and clearer, the Nährzelle somewhat 
smaller and more granulär. These cells always separate from the 
ovary and pass out into the body-cavity in pairs. At first the twin 
cells do not differ much in size; but the nucleus of the Nährzelle 
is richer in chromatin than the germinal vesicle. Soon the Nährzelle 
