32 Prof. T. H. Huxley on the Development o/Pyrosoma. 



attached. The sharp contour of the membrane of the germinal 

 vesicle is then easily seen to be covered by nothing but a very 

 thin continuation of the pale delicate epithelium of the ovisac 

 which invests it and holds it in place, just as the discus proli- 

 gerus holds the human ovum in its place in the Graafian 

 follicle. 



In the earlier stages, I have turned the ovum out of the ovi- 

 sac with its yelk quite entire ; so that there can be no question 

 here of destruction by manipulation. 



As the ovisac enlarges (up to ^th of an inch), the germinal 

 vesicle and spot retain their previous size ; but the contents of 

 the vesicle, instead of remaining transparent, become thick and 

 troubled, and acquire a yellowish colour and a certain opacity, 

 so as very nearly to resemble the previous yelk in consistence. I 

 have observed that this deposit appears to commence on that 

 side of the germinal vesicle which is turned towards the duct, in 

 consequence of which, the membrane of the vesicle is apt to 

 become shrunken and corrugated on the opposite side; so that, 

 viewed under a low power, it may appear truncated on that side. 

 Ultimately the whole vesicle is converted into a solid-looking 

 flattened mass, in which the germinal spot is perfectly distin- 

 guishable, while the membrane of the vesicle is hardly visible as 

 a distinct structure. The altered vesicle is now invested on its 

 inner side by the epithelium, while externally it is in contact 

 with the membrana propria of the ovisac. 



There is now a slight gap (which I hope yet to be able to fill 

 up satisfactorily) in my observations. Ovisacs of y\jth of an inch 

 in diameter no longer show the solidified germinal vesicle ; but 

 occupying exactly the same place is a thin discoidal oblong 

 body, about as wide as the solidified germinal vesicle, and of the 

 same colour, but between two and three times as long. In this, 

 no trace of the germinal spot is visible ; but it is composed of 

 minute celliform bodies, with distinct endoplasts or nuclei, of 

 the same aspect as the germinal spot, though much smaller. 

 The celliform bodies are above T ^Vo^ ^ an mcn m dia- 

 meter, the endoplasts -goVo-Toifoot ' while the germinal spot 

 attains its full diameter of about 2 uW^ of an inch in very young 

 ova, and neither increases nor diminishes in older ones. 



This is the earliest rudiment I have discovered of the blasto- 

 derm. The ducts of ovisacs of this size have a shrunken, withered 

 look ; and if there are spermatozoa in them, they are few and not 

 aggregated into a plug or bundle : on the other hand, scattered 

 about over the surface and on the face of the blastoderm, I 

 observe a number of minute rod-like solid bodies, curiously 

 similar to the heads of the spermatozoa. 



Simultaneously with, or slightly anterior to, the formation of the 



