640 



embryology: frog 



ectoderm, and becomes the lens, after passing through a stage 

 in which it is a hollow vesicle. 



Fig. 499. — A, B, and C, transverse sections through the ventral wall of the throat 

 of frog embryos of different ages, showing successive stages in the development 

 of the heart. — From Bourne. 



ep., Epiblast ; hy., hypoblast ; mes., mesoblast ; eth., endothelial lining of heart ; ht., heart ; pc, pericardial 

 cavity ; s., sucker ; so., somatic layer of mesoblast ; spL, splanchnic layer of mesoblast. 



ALIMENTARY CANAL 



The alimentary canal arises from three rudiments : the stomo- 

 daeum (p. 637) or fore-gut, which is of ectodermal origin and forms 

 the mouth ; the mesenteron or mid-gut, which is endodermal 

 (p. 634) and forms the greater part of the canal ; and the ecto- 

 dermal proctodaeum (p. 636) or hind-gut, which forms thecloacal 

 opening. The pituitary body arises by an outgrowth from the 

 roof of the mouth, meeting the brain-floor (p. 368). The gill 

 slits are formed by outgrowths from the endodermal pharynx, 

 which meet and perforate the skin. The first of them, correspond- 

 ing to the spiracle of the dogfish, never opens, but forms the 

 tympanic cavity and Eustachian tube.^ Between, in front of, 

 and behind the clefts mesodermal thickenings constitute the 

 visceral arches, in which skeletal and vascular structures corre- 

 sponding to those of the dogfish arise. The liver, pancreas, and 

 lungs arise as ventral outgrowths from the gut. The thyroid 



1 This is strictly true of many vertebrates (for instance, of birds and mammals), 

 but in the tadpole the first cleft disappears and where it stood the Eustachian 

 tube subsequently develops. 



