AND DEVELOPMENT OF PYROSOMA. 213 
for, while no fecundated female Ascaris which I examined would have failed 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 deprived 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 mass 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 by 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, 581), affirms that the germinal vesicle of the impregnated v^ of 
this worm bursts, and sets free the germinal spot, which is directly transformed into the 
first embryo-cell. 
The deservedly great authority of the late Johannes Miiller may be cited on the same 
side—so far, at least, as that singular mollusk, JSntoconcha mirabilis, is concerned. 
Dr. Gegenbaur affirms the occurrence of a similar process to be the rule among the 
Calycophoridce, Fhysophoridce, and certain other Ilydrozoa, and in that singular annulose 
animal, Sagitta. Thus, in describing the development of Oceania annate (Zur Lehrc 
vom Generationswechsel, 1854, 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 development 
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 zau naheren Kenntniss der Schwimmpolypen (lb5 4), 
the same author remarks, in giving an account of the development of these Calycophondce 
and J?hysophoridce 
a 
A process which may be here traced with particular clearness is the 
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 ol 
For 
c 
yelk-masses. I observed this process of yelk-division in Agalmopsis, Fhysoph 
skalia, Hippopodiits, and Diphyes, without noticing any important differences amon 
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 f I 
homogeneous, clear nucleus of the ripe ovum (the germinal vesicle) stood in a genetic re 
* « 
Ueber den Bau unci die systematise!* Stellung der Raderthiere." Siebold und Kolhker s Zeitseh; 
