EARLIEST MAMMALIAN GERM-PROCESSES. 1 87 



differ from the correspondingly developed embryos of other 

 higher Mammals (Plate VII., 1st row), we may quite safely 

 assume that the earliest germinal-processes, the cleavage of 

 the egg and the formation of the germ-layers, also corre- 

 spond. As yet, however, these processes have not been 

 actually observed ; for there has never been an opportunity 

 of dissecting a female of the human species immediately 

 after fertilization is completed, and of seeking the parent- 

 cell, or the cleavage-cells, in the oviduct. As, however, the 

 youngest human embryo (in the form of germ-vesicles), 

 which have yet been really observed, as well as the subse- 

 quently developed germ-forms, correspond in all essential 

 points with those of the Rabbit, the Dog, and other higher 

 Mammals, no reasonable man can doubt that egg-cleavage 

 and the formation of the germ-layers proceeds, in the one 

 case as in the other, in the way represented in Plate II. 

 Fig. 12-17. 57 



The particular form which egg-cleavage and the forma- 

 tion of the germ-layers assume in the case of Mammals, is, 

 however, by no means the original, simple, and palingenetic 

 form of germination. On the contrary, it has been very 

 much changed, vitiated, and kenogenetically modified in 

 consequence of numerous embryonic adaptations. (Cf. p. 12.) 

 It is, therefore, impossible from a mere study of it to learn 

 its nature. On the contrary, in order to obtain this know- 

 ledge, it is necessary to study and compare the various 

 forms of egg-cleavage, and of the formation of the germ- 

 layers, which occur in the animal kingdom ; and it is 

 especially necessary to search for the original, palingenetic 

 form, from which the modified, kenogenetic form of germi- 

 nation of Mammals gradually arose at a much later time. 

 15 



