HOMOLOGY BETWEEN HUMAN AND MAMMALIAN GEKMS. 365 



unmistakable. (Cf. Plates VI. and VII., the second row.) 

 But in a somewhat earlier stage, in which the rudi- 

 ments of the limbs, the gill-arches, the sense-organs, etc.^ 

 are already present, we cannot yet recognize mammalian 

 embryos as such, nor can we distinguish them from the 

 embryos of Birds and Reptiles. If we go back to still earlier 

 stages of development, we are unable even to discover any 

 distinction between the embryos of these higher Vertebrates 

 and those of the lower, such as the Amphibia and Fishes 

 (Plates VI., VII., upper row). Finally, if we go still further 

 back, to the Construction of the body from the four 

 secondary germ-layers, we make the surprising discovery 

 that these same four germ-layers exist, not only in all 

 Vertebrates, but also in all the higher Invertebrates, and 

 that they are everywhere concerned in the same way in 

 forming the fundamental organs of the body. And if then 

 we inquire into the origin of these four secondary germ- 

 layers, we find that they develop from the two primary 

 germ-layers, which are identical in all animals, with the 

 exception of the lowest division, the Protista. (Cf. Figs. 

 23-28, p. 93.) Finally, we see that the cells, which compose 

 the two primary germ-layers, universally originate by 

 fission, from a single simple cell, from the egg-cell. 



It is impossible to lay too much stress on this remark- 

 able parallelism of the most important germ-conditions of 

 man and animals. We shall afterwards turn the fact to 

 account in support of the hypothesis of monophyletic 

 descent, i.e., the assumption of the common, single line of 

 descent of man and the higher animal tribes. It declares 

 itself in the very beginning of the individual development ; 

 in the cleavage of the egg-cell, in the formation of the 



