OHAPTEK XI. 



COMPARISON OF THE FORMATION OF THE GERMINAL 

 LAYERS AND OF THE EARLY STAGES IN TH 

 DEVELOPMENT OF VERTEBRATES. 



ALTHOUGH the preceding chapters of this volume contain a fairly 

 detailed account of the early developmental stages of different groups 

 of the Chordata, it will nevertheless be advantageous to give at 

 this place a short comparative review of the whole subject. 



In this review only the most important points will be dwelt upon, 

 and the reader is referred fur the details of the processes to the 

 sections on the development of the individual groups. 



The subject may conveniently be treated under three heads. 



(1 ) The formation of the gastrula and behaviour of the blastopore : 

 together with the origin of the hypoblast. 



(2) The mesoblast and uotochord. 



(3) The epiblast. 



At the close of the chapter is a short summary of the organs 

 derived from the several layers, together with some remarks on the 

 growth in length of the vertebrate embryo, and some suggestions as 

 to the origin of the allantois and amnion. 



Formation of the gastrula. Amphioxus is the type in which 

 the developmental phenomena are least interfered with by the 

 presence of food-yolk. 



In this form the segmentation results in a uniform, or nearly 

 uniform, blastospheie, one wall of which soon becomes thickened and 

 invaginated, giving rise to the hypoblast; while the larva takes the 

 form of a gastrula, with an archenteric cavity opening by a blasto- 

 pore. The blastopore rapidly narrows, while the embryo assumes an 

 elongated cylindrical form with the blastopore at its hinder extremity 

 (tig. lb'9 A). The blastopore now passes to the dorsal surface, and by 

 the flattening of this surface a medullary plate is formed extending 

 forwards from the blastopore (fig. 169 13). On the formation of 

 the medullary groove and its conversion into a canal, the blastopore 

 opens into this canal, and gives rise to a neurenteric passage, leading 



