226 PROFESSOR HUXLEY ON THE ANATOMY 



aperture of the duct ; and when the ovisac is opened, the germinal vesicle is found to 

 adhere to this point with considerable tenacity. It is, in fact, held in place by a conti- 

 nuation of the epithelial lining, which, lies between it and the cavity of the ovisac — the 

 germinal vesicle being now situated between the epithelium and the membrana propria, 

 so that while its outer face is covered by the latter its inner face is invested by the 

 former. All this will be rendered easily intelligible by examining the profile views (fig. 6 

 and 6«), and the view from within (fig. 7), of the germinal vesicle in situ. 



But it has been seen that the ovum, containing the germinal vesicle, originally lay 

 inside the wall of the ovisac, which has become metamorphosed into the epithelium, and 

 hence it follows that the germinal vesicle, after losing its yelk, must pass through the 

 epithelium of the ovisac. It will be recollected that the mammalian ovum becomes simi- 

 larly related to the epithelium of the Graafian follicle, and that the germinal vesicle of 

 the bird's egg in lilie manner passes into and through the peripheral layer of its yelk. 



Throughout the present stage there is not the least difficulty in observing the germinal 

 vesicle and its spot in the uninjured ovisac. The spot, in fact, is particularly well-defined, 

 and immediately strikes the eye when even a low magnifying power is used. But, with 

 such a power (say 200 diameters), it is easy to fall into error as to the shape of the 

 germinal vesicle. It constantly appears to be hemispherical, the truncated side being 

 that which is turned away from the upper aperture of the duct. This appearance arises 

 from the fact that, with such a power, one sees the contents and not the wall of the 

 germinal vesicle, and as the yellow deposit fills only that moiety which lies nearer the 

 upper aperture of the duet, it appears like a semicircular cake. Under a higher power 

 (500 diameters) the wrinkled* membrane of the other moiety of the vesicle is always 

 readily discernible. 



Fifth Stage. Ovisacs between -^th and-g^th of cm inch in diameter, in lohich the germinal 

 spot disappears, and a number of minute granules take its ptlace. 



The germinal vesicle, held in place on the neural wall of the ovisac, immediately behind 

 and rather to the right of the vipper aperture of the duct, in the manner which has just 

 been described, next undergoes changes of very great interest and importance. I have 

 devoted a very great deal of time and patience to the analysis of these changes, but it is 

 only recently that I have felt satisfied with the results of my investigations ; and I must 

 warn any one who is disposed to repeat these observations, that while everything which 

 I have described up to the present moment may be demonstrated with the utmost 

 readiness in almost any thin vertical section of the ascidiarium, the conditions of the 

 germinal vesicle which I am about to describe occur but very rarely, and require the aid 

 of high powers of a thoroughly good microscope for their complete elucidation. Out of 

 a vast number of preparations which I have made, at intervals, during the last twelve 

 months, not more than eight or nine have exhibited the exact features of which I am about 

 to give an account. 



The germinal vesicle represented in fig. 8«, Ijelongs, strictly speaking, to the preceding 



* The fact that the Pyrosoma ohserved by me had been preserved in spirit should always be recollected. It is 

 highly possible that the wrinklings are artificial. 



