Manchester Memoirs, Vol. I. (1906), No. 3. 6"] 



confined to a blastoderm, but in this we distinguish again 

 an upper and a lower layer of cells ; as far as their relative 

 positions are concerned, the upper layer corresponds very 

 closely with the animal hemisphere ; the lower layer, 

 together with the yolk, to the vegetative hemisphere in 

 the former case. There thus exists a very fairly complete 

 morphological similarity or homology between the primary 

 germinal layers throughout (we except, of course, the 

 Placental Mammals, in which any attempt to identify the 

 outer layer and inner mass with the primary germ layers 

 has long ago been abandoned), though it must be borne 

 in mind that this rests merely on the structure of the fully 

 segmented &^%, not on anything that we know of the 

 origin of these layers during segmentation. 



To the second question, however, is the relation which 

 these primary layers bear to the parts of the embryo in 

 the next stage constant and invariable, we can most cer- 

 tainly not return an affirmative answer ; for while in the 

 Anamnia the floor of the archenteron and the yolk-plug 

 are derivatives of the primary * endoderm,' in the Amniota 

 they are of purely 'ectodermal' origin ; nor is it even 

 possible to say that the cell-layer which forms the archen- 

 teric roof is homologous throughout, its ' ectodermal ' and 

 ^ endodermal ' origin having been equally maintained. 



If we prefer to compare the primary layers not with 

 the structures visible in the next stage, but directly with 

 the organization of the adult, our difficulties are just as 

 great. For now the alimentary tract of Teleostei (and 

 Elasmobranchii ?) is ectodermal, that of Ceratodus, 

 Gymnophiona, and Amniota ' endodermal,' that of the 

 Frog and Lepidosiren of mixed origin. It is abundantly 

 clear that each of the primary germinal layers, as defined 

 by Haeckel, though constant in its structural relations 

 ' throughout the series, may be, and often is in its destiny, 

 exceedingly diverse. 



