UNEQUAL VALUE OF THE "ANCESTRAL STAGES. 41 



parent-forms are of very diverse values in relation to the 

 certainty of our knowledge. From the few remarks which, 

 while speaking of the Ontogeny, we made as to the corre- 

 3]X>nding phylogenetic forms, it will have been understood 

 that some germ-forms may with certainty be regarded as 

 reproductions of corresponding parent-forms. We recog- 

 nized the human egg-cell and the parent-cell which results 

 from the impregnation of the latter as the first and most 

 important form of this kind. 



From the weighty fact that the egg of the human being, 

 like the egg of all other animals, is a simple cell, it may be 

 quite certainly inferredtiiat a one-celled parent-form once 

 existed, from which all the many-celled animals, Man in- 

 cluded, developed. 



A second very significant germ-form, which evidently 

 reproduces a primaeval parent-form, is the germ-vesicle 

 ' 'Blastula), a simple hollow sphere, the wall of which con- 

 sists of a single cell-stratum. A third extremely import- 

 ant form in germ-history, which may be quite safely and 

 directly referred back to the tribal history, is the- true Gas- 

 trula. This most interesting larval form already exhibits 

 the animal body composed of two germ-layers, and fur- 

 nished with the fundamental primitive organ, the intestinal 

 canal. Now, as the same two-layered germ-condition, with 

 the primitive rudiment of the intestinal canal, is common to 

 all the other animal tribes (with the single exception of the 

 Primitive Animals, Protozoa), we may certainly from this 

 infer a common parent-form of similar construction to the 

 Gastrula, the Gastrsea. Equally important in their bearing 

 on the Phylogeny of Man, are the very important ontoge- 

 netical form conditions which correspond to certain Worms, 



