CHAPTER XI. 



COMPARISON OF THE FORMATION OF THE GERMINAL 

 LAYERS AND OF THE EARLY STAGES IN THE 

 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 advan- 

 tageous 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 for 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 notochord. 



(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, blastosphere, 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 blastopore. The blastopore rapidly narrows, while the 



1 8 2 



