CH. XIII] ORGAXS FROM THE EXDODERM 141 



anterior portion into the yolk-mass behind (Fig. 37, A, B) 

 forms the beginning of the liver. 



The first gill-slits appear at a stage when the medullary folds 

 have rolled over and are about to fuse. At the present stage, 

 the gill-slits are well marked. They appear along the lateral 

 walls of the enlarged anterior end of the archenteron as solid 

 outgrowths of its wall. At the posterior end of the archenteric 

 cavity the position of the blastopore, which has now closed, 

 is marked by a diverticulum, the so-called "post-anal-gut" 

 (Fig. 37). It is in this region that the neurenteric canal of 

 the embryo persists for a short time after the blastopore has 

 been covered over by the medullary folds. The pit-like invagi- 

 nation of ectoderm, the proctodseum, has opened into the pos- 

 tero-ventral portion of the archenteron (Fig. 37, B). 



At the time when the tadpole is ready to emerge from the 

 jelly-capsule (Fig. 38), the anterior portion of the archen- 

 teron has become larger and longer (Fig. 39), and in the re- 

 gion where the heart forms, ventral to the pharynx, an inward 

 projection of the endodermal wall is present. In the middle 

 region of the embryo the lumen of the archenteron is reduced 

 to a small cavity, as seen in cross-section (Fig. 40), and is now 

 longer from above downward than from side to side. The 

 yolk-mass as a whole is rounded and more compact than in the 

 earlier stages. At the posterior end of the embryo the archen- 

 teric cavity bends around the end of the yolk-mass, taking a 

 curved course to open on the ventro-posterior surface of the 

 body by the anus. 



During the early stages of development the cells of the em- 

 bryo have been exceedingly active, but no food has been taken 

 as yet into the digestive tract, for the mouth does not open 

 until some time after the embryo has left the egg-membranes. 

 All the cells of the body contain yolk-granules, which serve in 

 part, beyond doubt, to supply the energy necessary for develop- 

 ment. A large amount of yolk is also stored up in the endo- 

 derm cells of the ventral yolk-mass, and must also long serve 

 as a source of nourishment for the young tadpole. 



The changes in shape that the archenteron passes through 

 seem to be in part a result of the activity of the endodermal 

 cells, and in part the necessary result of the change in shape 



