ORGANIC EVOLUTION 



197 



The archenteron is converted into an alimentary canal by a 

 process somewhat differing from that 

 followed in the worm ; it is destined 

 to occupy a reversed position; . the 

 blastopore becomes the anus, and the 

 mouth is formed at the opposite 

 end by an ingrowth from the ecto- 

 derm that meets the front end of the 

 archenteron and fuses and then opens 

 a passage through. The anterior 

 end of the archenteron becomes dor- 

 sally flattened and laterally ex- 

 panded into a pharynx, from whose 

 walls sacculations of endoderm grow 

 outward to meet the ectoderm, and 

 then cleave apart on vertical lines, 

 opening gill clefts on the side of the 

 neck. The pillars of tissue left 

 standing between these clefts become 

 the gill arches. By these simple 

 processes, are laid down the main 

 lines of vertebrate structure. 



Endodermal differentiation. The 

 embryonic endoderm becomes 

 epithelium, as before, and is the lining 

 layer of the alimentary canal and of its appendages. It is 

 for the most part a single layer of cells not remarkably modi- 

 fied in form, differing in length according to the extent of 

 their compression. In the pharynx they are short-cylindric 

 and ciliated on their free ends (figure 134) . In the intestine 

 they are much more compressed and slender, and certain of 

 their number are differentiated as goblet-shaped mucus- 

 secreting cells. In the stomach where the wall is made up 

 of multitudinous pit-like depressions called the gastric 



FIG. 122. Diagram of the 

 formation of neuron, no- 

 tocord and coelom. The 

 ectoderm is white, the 

 endoderm is solid black 

 and the mesoderm is 

 crosslined ; c c, notocord ; 

 c n, pronephric duct. 



