830 BIOLOGY. [BOOK iv. 



all fishes which differentiate themselves; then reptiles, then 

 birds, then mammifers ; finally the diverse mammiferous types. 

 The less individuals are to differ in the adult state, the more 

 tardily their embryons differentiate themselves. It is at the 

 last period of development, for instance, that the embryon of a 

 monkey can be distinguished from the embryon of a man, and 

 there is a moment when both do not differ more from the em- 

 bryons of a fowl or of a tortoise than they differ from each 

 other. 1 It is difficult not to see here, as the partisans of evolu- 

 tion are inclined to see, a sort of living palaeontology, an abridged 

 picture of the formation of diverse animal types, such as has 

 been accomplished in the course of the cycles which have rolled 

 away. 



The development of the fecundating element is at first so 

 analogous to that of the ovulum, properly so named, that it has 

 not unjustly been called the male ovulum. Wherever sexuate 

 reproduction exists in the two kingdoms, the male element is 

 primitively a complete cell, containing a protoplasm comparable 

 with the vitellus and a nucleus analogous to the germinative 

 vesicle. 



These male ovula, according to the testimony of many ob- 

 servers, seem usually to be formed by spontaneous genesis, as 

 well in the anther of phanerogamic giants, as in the special 

 apparatus of animals. 



Once the male cell formed and arrived at a certain degree of 

 maturity, its granulous protoplasmic content undergoes, spon- 

 taneously, a phenomenon, which vividly recalls the fractionment 

 of the fecundated female ovulum. The last elements of that 

 f ractionmeiit, which are generally in an even number, resemble 

 each other much in most of the cryptogamic plants, and in all 

 the animal kingdom. They are cells, whence comes forth finally 

 a mobile corpuscule, called spermatozoid in the vegetal kingdom, 

 and spermatozoary, or zoosperm, in the animal kingdom. The cells 

 of fractionment, each of which produces, as a male, only one of 

 1 Huxley, Man's Place in Nature. 



