Manchester Memoirs, Vol. I. (1906J, No. 3. 63 



derm (Amniota). The dorsal lip is always 

 formed at the posterior margin. 



{b) In all Anamnia, except the Gymnophiona, 

 the ventral lip is developed immediately (microle- 

 cithal forms), or after a considerable interval 

 (megalecithal forms), from the anterior margin 

 of the blastoderm. 



(c) In the Gymnophiona and Amniota the 

 ventral lip is always developed by the fusion of 

 the lateral lips, the anterior margin of the blasto- 

 derm (or embryonic shield) taking no part 

 whatever in the process. 



(7) Considerable discrepancies exist in the manner in 

 which the endoderm is formed in different types. 

 This may arise from the roof only, the floor 

 only, or from both roof and floor of the archen- 

 teron, or from the paraderm. 



It is these anomalies which lead us naturally to a 

 critical consideration of the morphological value of the 

 germinal layers. 



Part II.— THE MORPHOLOGICAL SIGNIFI- 

 CANCE OF THE GERMINAL LAYERS. 



The phylogenetic speculations which have so long 

 dominated our conceptions of development are all directly 

 traceable to a hypothesis first enunciated by Ernst 

 Haeckel, thirty years ago. This hypothesis is the 

 ' Gastraea-Theorie.' 



There occurs, according to this theory, in the ontogeny 

 of all multicellular animals, a two-layered form, the Gastrula, 

 produced by invagination or some modification of that 

 process from a one-layered form, the Blastula. The two 



