THE DIGESTIVE AND RESPIRATORY SYSTEM 259 



ture soon disappears, leaving the gastroccele as a closed sac,' 

 with neither oral nor anal orifices. These are formed secon- 

 darily through inpushings of the ectoderm, the blind ends of 

 which come in contact with the endoderm and later break 

 through at the point of contact, thus completing the canal 

 (Fig. 71, B, in and an). The functional alimentary canal 



B 



FIG. 71. Diagrams showing the formation of the vertebrate alimentary 

 canal and nerve cord, and the early relation between them. 



(A) Early embryo, immediately after the gastrular stage, based on Amphioxus. 

 Compare this with Fig. 13 (c). (B) later stage, based on the embryo of the frog. 



g, gastroccele (=cavity of alimentary canal); n, neurocosle (=<cavity of the 

 neural tube); ne, neurenteric canal; np, neuropore; b, blastopore; an, developing 

 proctodseum; m, point where the stomatodaeal invagination will take place; d, liver 

 invagination; h, heart; nc, notochord. 



thus comes to be formed of three elements, an anterior ecto- 

 dermic one, the stomatodccum, a middle endodermic one, the 

 mesodcntin, and a posterior portion, also ectodermic, the 

 proctod&um. In the articulates (crustaceans, insects, and 

 spiders), in \vhich the alimentary canal is formed in much 

 the same way, the proportion of the functional digestive tract 



