AND DEVELOPMENT OF PYROSOMA. 24-3 



for, while no fecundated female Ascaris which I examined would have faUed to supply 

 me with a complete series of all the other stages of development, it was but thus rarely 

 that these first processes presented themselves. 



" As the ovum, now dejirived of its germinal vesicle and spot, is propelled downwards 

 by the peristaltic contractions of the uterus, the first embryo-cell is formed in the middle 



of its clear yelk. I have never been able to detect the mode of its origin By 



endogenous development, the embryo-cells give rise to other cells, which become the 

 blastodermic mjiss whence the embryo is formed. The yelk, as such, disappears." 



I am prepared to admit the full force of this carefully observed example of the disap- 

 pearance of the germinal vesicle and the merging of its contents in the yelk, but it is the 

 only case, within my knowledge, to which great weight can be attached ; while, on the 

 other hand, independent observers have (of late years) recorded equally definite and 

 positive observations that in some groups of animals, at any rate, the germinal vesicle 

 does not disappear, but that it gives rise l^y division to the primary cells of the embryo. 



Thus, Dr. Nelson, in his memoir "On the Reproduction of Ascaris mystax" (Phil. 

 Trans. 1852, pp. 580, 681), afiirms that the germinal vesicle of the impregnated egg of 

 this worm bursts, and sets free the germinal spot, wliich is directly transformed into the 

 first embryo-cell. 



The deservedly great authority of the late Johannes MliUer may be cited on the same 

 side — so far, at least, as that singular moUusk, Eidoconclia mirabilis, is concerned. 



Dr. Gegenbaur afiirms the occurrence of a similar process to be the rule among the 

 Calycoplioridce, Tliysophoridce, and certain other Hydrozoa, and in that singular annulose 

 animal, Sagitta. Thus, in describing the development of Oceania armata (Zur Lehre 

 vom Generationswechsel, 1851, p. 28), Gegenbaur says (the italics are his own) : — 



" Every act of division is preceded by a division of the nucleus, and consequently the 

 first act by the division of the germinal vesicle : the transparency of the yelk allows of 

 the most precise observation of all these phenomena, and the following of the develop^nent 

 of the nuclei of the later embryo-cells out of the original germinal vesicle (the nucleus of 

 the primitive ovi-cell)." 



Again, at p. 50 of his "Beitrage zur naheren Kenntniss der Schwimmpolypen " (1851), 

 the same author remarks, in giving an account of the development of these Calycophoridce 

 and JPhysophoridce : — 



" A process which may be here traced with particular clearness is the constant division 

 of the germinal vesicle,' which precedes the division of the yelk ; and the products of the 

 division of the germinal vesicle behave similarly, in relation to the subdivision of the 

 yelk-masses. I observed this process of yelk-division in Agalmopsis, Physophora, For- 

 skalia, I-Iip)popodins, and Biphyes, without noticing any important differences among 

 them." 



Leydig expresses the same conclusion, though more guardedly, in his account of the 

 development of the ova of Notommata Sieboldii * : — 



" The nuclei of the division-masses are very clear ; and it appeared to me as if the 

 homogeneous, clear nucleus of the ripe ovum {the germinal vesicle) stood in a genetic rela- 



* " Ueber den Bau und die systematische Stellung der Raderthiere." Siebold und Kiilliker's Zeitschrift, 1855. 



2 K 2 



