IN THE EMBRYOGENY OF THE HIGHER ANIMALS. 157 



2. We may consider first the phenomena of embry- 

 ogeny as presenting a certain parallelism with the (proto- 

 morphic) gemmation which occurs in some of the lower 

 species (Trematoda, Mosses, &o.) before the period when 

 the typical form is attained. 



No point in embryogeny is better established than this, 

 that the first result of impregnation is the formation in the 

 ovum of a cellular mass, from one point of which is subse- 

 quently developed a fresh axis of growth, destined for evolu- 

 tion into the organization typical of the species, while the 

 original germ-mass disappears as a distinct structure. The 

 embryo, in short, may be said to be budded off from the 

 primordial germ-mass or " mulberry body," much as the 

 cercariform larva of the Distoma is from the gregariniform 

 product of the Trematode ovum. 



There are, however, two striking points of diversity 

 which must not be overlooked. In ordinary and normal 

 development the original germ-mass gives rise only to a 

 single embryo, and no separation takes place between them ; 

 the later growth appears simply as a more advanced state 

 of the former, which wastes away, pari passu, with the 

 growth of the embryo, becoming a mere appendage of the 

 latter, or, it may be, disappearing altogether. In the cases 

 again of alternation of generation, which we have had under 

 review, the immediate product of impregnation gives origin 

 to numerous gemmae, every one of which may acquire the 

 characters of a typical individual of the species. We find 

 also that these gemmae generally become completely sepa- 

 rated from their germ-parent, and assume the form of inde- 

 pendent organisms. 



3. But although the detachment of the later growth, 

 and its multiplication, give an apparent distinctness to the 

 cases in which they occur, they can scarcely be considered 

 as characters of sufficient constancy to establish, of them- 

 selves, an essential diversity of nature between them and 



