ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 379 
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 endo- 
genous 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 disappearance of the germinal vesicle and the merging 
of its contents in the yelk, but it is the only case, within my know- 
ledge, 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 egg 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, 
Entoconcha mirabilis, is concerned. 
Dr. Gegenbaur affirms the occurrence of a similar process to be 
the rule among the Calycophoride, Physophoride, and certain other 
Hydrozoa, and in that singular annulose animal, Sagztta. Thus, in 
describing the development of Oceania armata (Zur Lehre vom Gene- 
rationswechsel, 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 dvzszon 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 nuclet 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” (1854), the same author remarks, in giving an 
account of the development of these Calycophoride and Physo- 
phoride :— 
“ A process which may be here traced with particular clearness 
is the constant division of the germinal vesicle, which precedes the 
