62 Jenkinson, Germinal Layers of Vertebrates. 



The broad features of germ-layer development in the 

 whole group of Vertebrata may now be summed up in 

 the following formulae. 



(i) By the germinal layers are to be understood the 

 ectoderm — the source of the epidermis and its 

 derivatives, of the nervous system and the organs 

 of special sense ; the endoderm — or lining 

 epithelium of the alimentary tract and its 

 diverticula ; and the mesoderm (including the 

 notochord) — the layer which gives rise to the 

 musculature, the skeletal and connective tissues, 

 the vascular system and the urogenital organs. 



(2) The material for the ectoderm, notochord and 



mesoderm, and roof of the archenteron is brought 

 into position during an overgrowth and in- 

 growth of cells which takes place at the lip of 

 the blastopore during the closure of the latter. 



(3) This closure is bilaterally symmetrical, taking 



place more actively at the dorsal lip than at any 

 other point, and leads to the formation of a 

 bilateral archenteric cavity, the extent of which 

 is greatest anteriorly and least posteriorly, 



(4) At the lips of the blastopore the ectoderm, the 



roof of the archenteron, and the notochord and 

 mesoderm are continuous with one another. 



(5) The notochord lies medially in front of the dorsal 



lip ; the mesoderm sheets spring from the lateral 

 lips and pass into one another posteriorly behind 

 the ventral lip. 



(6) {a) The blastoporic lips are formed from the 



margin of the blastoderm ^Anamnia) or of an 

 embryonic shield lying wholly within the blasto- 



