4.8 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 certain 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), 8 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 Gsh, an amphibian 



