68 jENKlNSOlsi, Ger;/iiua/ Layers of Vertebrates. 



But, it will no doubt at once be said, in the Vertebrates 

 the Gastrula must be looked for, not at the end of seg- 

 mentation, but in that stage in development when the 

 blastopore is already closing. Though this is neither 

 Haeckel's two-layered Gastrula (except in the case of the 

 * Disco-Gastrula ' of the Teleostei), nor the Planula of 

 Lankester, we will^nevertheless examine the hypothesis in 

 its new form. In this stage the Vertebrate embryo 

 possesses a blastopore leading into an archenteron, the 

 archenteron has a floor and a roof, and from the lips of the 

 blastopore the notochord and mesoderm have been pro- 

 duced between the archenteron and the ectoderm. There 

 is a very close similarity between all embryos at this stage^ 

 and we may — if we disregard for the moment the 

 differences in their mode of origin — fairly describe their 

 parts as homologous. But do these homologous parts 

 exhibit that absolute constancy in their relation to adult 

 structures which the theory demands ? By no means ; 

 the ectoderm truly has the same fate in all cases, except 

 when, as in the skin glands of Amphibia, it gives rise to 

 muscles, or, as in the thymus of Elasmobranchs, to blood- 

 corpuscles, and so has the^mesoderm, except when it gives 

 rise to enamel ; but the roof and floor of the archenteron 

 vary widely in their subsequent behaviour. The roof may 

 form notochord only, or notochord and mesoderm only, 

 or both these, and either the roof or the whole of the 

 definitive gut. The floor may form a part or the whole of 

 the lining epithelium of the digestive tract, or be 

 excluded from it altogether. It will be urged, of 

 course, that the archenteric cavity nevertheless becomes 

 the alimentary canal ; but here the Amniota involve 

 us in great difficulties ; so little a part does the archen- 

 teron play in the formation of the lumen of the gut that 

 Hertwig has felt constrained to call it a ' Mesodermsack- 



