48 THE DOCTRINE OF DESCENT. 
on the contrary, the unfertilized eggs produce females 
only; and it is the same with several of the lower 
crustaceans. 
We will now revert to the consideration of the evolu- 
tionary processes displayed in sexual reproduction after 
fecundation has taken place. Development invariably 
commences with a process of cell-formation, the bifurca- 
tion or formation of the germinal membrane, after the 
completion of which, instead of the one primitive cell, 
a large number of cells are usually in existence, as the 
material for the distribution and construction of the 
embryo. Ova developing parthenogenetically, without 
fecundation, likewise commence their development by 
this multiplication of cells; and even the ova of ani- 
mals, in which development never takes place without 
previous fecundation, exhibit an incomplete bifurcation, 
if not fertilized at a ccrtain stage of maturity. This 
process, it is true, has been as yet demonstrated only in 
the ova of the frog and the domestic fowl ; but these 
cases are sufficient to divest the bifurcation of the 
character of an independent phenomenon, exclusively 
restricted to sexual reproduction. 
Even before the appearance of C. E. von Baer’s really 
classical and fundamental work on the “ Evolutionary 
History of Animals” (Entwickelungsgeschichte der 
Thiere),’ the view, founded on incomplete observations, 
had become established, that in the various stages of 
their development the higher animals passed through 
the forms of the lower ones. In this, natural philosophy 
did not confine itself to the limits of the types; and 
hence did not pause at the hypothesis that the mam- 
malian embryo was successively a fish, an amphibian, 
