SIXTEN BOCK, TWO NEW COTYL. GEN. OF POLYCLADS. 9 



germinative zone reaches only 30 »j-, while in the same ovary 

 the single large maturing oocyte can be more than Y* nfi^a. 

 in diameter. Thus the latter oocyte surpasses the whole 

 germinative layer more than 20 times in volume. I have 

 never seen anything like this in other polyclads, where the 

 ovaries usually contain neat series of oocytes in different 

 stages of maturity. It is, indeed, very peculiar to see such 

 an ovary, containing a small number of young ovarial cells 

 of the same stage beside the single, large, maturing »egg». 

 The production of such large eggs as these of Chromojylana 

 can be established only at the expense of the number. But 

 not only has the number of eggs produced by each ovary 

 been reduced, but also, as I mentioned above, the total number 

 of ovaries. 



Each ovary is surrounded with a special muscle cover 

 outside the membrana propria. In the larger ovaries which 

 contain fully developed oocytes this muscle sheath is less 

 conspicuous on account of the muscle fibres being extremely 

 extended, but when an ovary has given off the large oocyte, 

 the muscle envelope contracts to great thickness around the 

 remaining immature oocytes. Such a strong muscle cover of 

 the ovaries I have hitherto not met with in the polyclads 

 nor is it described for this group. Its function is not only 

 to drive out the mature oocytes but also, which can be 

 easily ascertained in my specimens, to eliminate the space 

 left by the departed oocyte and thus bring the remaining 

 ovarial cells in closest contact with the surrounding nourish- 

 ing tissue. The large size of the ova, which, for instance, 

 surpasses that of the 15 times lon ger Prosthecerceus vittatus 

 (MoNT.) many times in volume, might make such a strong 

 musGulation necessary, but I find it more remarkable as the 

 musculation of the body-wall and most of the organs is 

 particularly feebly developed. 



I have failed to observe any uteri at all and I am unable 

 to say if this is due to the specimens not having yet reached 

 full female maturity or because the uteri really have been 

 reduced. I am inclined to take the latter view as the ovaries 

 generally contain eggs which have attained full maturity and 

 are ready for impregnation and deposit. If this reduction 

 of the uteri actuall}^ has taken place, and there is much in 



