EARLY DEVELOPMENT OF AMPHIOXUS. 



entoderm constitutes the primitive gut, the only opening of which is the 

 blastopore, serving as both mouth and anus. Already the simple alimentary 

 system is confined to the interior of the organism, shut off from the outside 

 except through an opening for the intake of food and output of waste. 

 Among the invertebrates the sponges and corals never develop beyond the 

 two layered, or didermic, gastrula stage such as we here see in Amphioxus. 

 It is worth noting also that in Amphioxus the cells with yolk content are 

 members of the entoderm group; in other words, a temporary food supply, 



Notochord 



Neural 

 plate 



Ectoderm 

 Neural plate 



Ccelom 



Intestine 



Entoderm. 



Parietal 

 mesoderm 

 Visceral 

 mesoderm 



Intestine 

 Entoderm 



FIG. 22. From transverse sections through Amphioxus embryos, showing successive stages in 

 formation of mesoderm, neural tube and notochord. Bonnet. 



scanty as it is here, is stored in the lining of the gut. From this simple primi- 

 tive gut the whole alimentary system is elaborated, complex as it may 

 become. The mouth, however, is not a derivative of the blastopore, but 

 develops as a new opening into the cephalic end of the gut cavity. The 

 anal opening too in most vertebrates arises independently. 



Before considering the formation of the middle germ layer, or mesoderm, 

 it is desirable to observe certain changes affecting the exterior of the gastrula 

 which are correlated with the development of the nervous system, because 

 they occur prior to the appearance of the mesoderm and produce a setting 

 for part of this layer. Along the flattened dorsal surface of the gastrula a 



