EMBRYONIC DEVELOPMENT 



209 



divided into two layers, one applied to the inside of the ectoderm, the 

 other covering the endoderm. The peritoneum, which occupies approxi- 

 mately the corresponding positions in later stages, is derived from these 

 layers. Above the endoderm, between it and the notochord, two layers 

 of the mesoderm approach one another and form the mesentery (Fig. 176, 

 page 205) which later suspends the digestive tract in a trough of peri- 

 toneum. In the longitudinal section (Fig. 183) the mesoderm is not 

 represented above the digestive tract, since the section passes exactly 

 through the median plane. But below the intestine the mesoderm occurs, 

 divided into its two characteristic layers. 



Fig. 182. — Cross section of the early embryo of a frog, diagrammatic, c, coelom; dig, 

 digestive tract; ec, ectoderm; en, endoderm; ms, mesoderm; nc, neural crest; nd, notochord; 

 nf, neural fold; ng, neural groove. 



Anterior Digestive Tract. — The gill pouches, represented as seen from 

 above in Fig. 184, are evaginations of the endoderm in the sides of the 

 pharynx, or anterior part of the gut. Typically there are five of these 

 protrusions on each side, but some of them are often rudimentary, or 

 two of them may be nearly combined, so that the number frequently 

 appears to be less. Successive stages in the evagination of the gill 

 pouches are shown in A, B, C. They finally reach the ectoderm, with 

 which they fuse. In fishes and usually in amphibians the ectoderm 

 and endoderm both break open at the point of fusion, so that the pharynx 

 is open to the outside. These openings are the gill clefts. They serve 

 as channels for the passage of water, which enters at the mouth (not 

 shown in the figure since it is at a lower level). The course of the water 



