242 PROFESSOR HUXLEY ON THE ANATOMY 



among the cells of the proligerous disk. A still closer parallel, perhaps, is presented by 

 the bu'd's egg, if we consider the mode in which its germinal vesicle (which at first occu- 

 pies the centre of the future egg, and is contained in a primitive ovum surrounded by, at 

 any rate, a rudimentary vitelline membrane) passes to the surface, and eventually lies 

 immediately beneath the membrane which encloses the food-yelk *. 



5, 6, 7. The consideration of the phenomena enumerated under these heads opens up 

 the whole vexed question of the fate of the germinal vesicle. 



Since the imaginations of Dr. Martin Barry have fallen into just discredit, most 

 physiologists have more or less distinctly adopted the doctrine that the germinal vesicle 

 and its contents lose their identity and disappear ; and that the embryo-cells, whence 

 the blastoderm arises, are new structures not directly derived from them. 



The evidence by which this conclusion is supported, however, will be found, if closely 

 sifted, to be, for the most part, not only negative, as by the nature of the case it must be, 

 but weakly negative. That is to say, not only is the conclusion based upon the circum- 

 stance that, at a given period, the observer was unable to find the germinal vesicle or to 

 identify its contents, — but, in most cases, the circumstances are such that he might very 

 well have missed them had they existed. Even in Fyrosoma it is no easy matter, until 

 one has had some practice, to find the germinal vesicle Avhen it is passing into the 

 blastoderm, although, in all the earlier stages, nothing can be more obvious ; and had 

 the ovisac of Fyrosoma been filled with even a very slightly granular yelk, I believe the 

 discovery of the germinal vesicle, at this period, would be almost impracticable. What 

 wonder, then, that it should be impossible to identify the germinal vesicle or its contents 

 in the midst of the more or less opake and coarsely grantilar substance of which the yelk 

 of ninety-nine ova out of a hundred is composed ? The only case to which this reasoning 

 does not apply is that described by KoUiker in the paper already referred to (/. c. p. 76) : — 



"As regards the internal changes undergone by the eggs [oi Ascaris dentata], the most 

 striking fact is that, immediately after fecundation, the germinal spot and the germinal 

 vesicle have disappeared and the clear and transparent yelk contains notliing but scanty 

 elementary granules. This is a point of great importance ; and to show that there is no 

 possibility of being deceived about it, I add, that the ovum of Ascaris dentata, including 

 its chorion and vitelline membrane, is so transparent that all the outlines of a body 

 which may happen to lie beneath it are quite sharply and distinctly recognizable, and its 

 contents are so clear and patent that hardly the smallest elementary granule of the yelk 

 can remain hidden. Which of the two parts first disappears, the germinal spot or the 

 germinal vesicle, I cannot as yet say with certainty ; but, in one individual, I saw two 

 ova which had hardly traversed the seminal cells in the fundus uteri, and though they 

 still exhibited a germinal vesicle, had no germinal spot. In another individual, I 

 observed the same thing in an ovum imbedded in the midst of the seminal cells; 

 so that I have some ground for the opinion that it is the germinal spot which disappears 

 first. Further and repeated observations must decide whether this is the rule or whether, 

 in other cases, it is not the germinal vesicle which disappears first. But I must observe, 

 that this first stage of the development of the ova appears to be of very short duration ; 



* See Dr. A. Thomson's admirable article " Ovum " in Todd's Cyclopjedia. 



