EMBRYOLOGY 



497 



The alimentary canal arises from three rudiments : the 

 stomodseum or fore-gut, which is of epiblastic 

 canT ary origin and forms the mouth ; the mesenteron or 

 mid-gut, which is hypoblastic and forms the 

 greater part of the canal ; and the epiblastic proctodeum or 

 hind-gut, which forms the 

 cloacal opening. The pituit- 

 ary body arises as an out- 

 growth from the roof of the 

 mouth. The gill-slits are 

 formed by outgrowths from 

 the (hypoblastic) pharynx, 

 which meet and perforate 

 the skin. The first of them, 

 corresponding to the spiracle 

 of the dogfish, never opens, 

 but forms the tympanic cavity 

 and Eustachian tube. Be- 

 tween, in front of, and behind 

 the clefts mesoblasticthicken- 

 ings 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 body starts 

 as a median longitudinal 

 groove in the floor of the 

 pharynx. This gives rise to 

 a solid mass of cells which 

 separates from the pharynx 

 and divides into two. The 

 intestine of the tadpole, 



when the yolk in its ventral wall has been absorbed, 

 becomes for a time more coiled than that of the adult frog, 

 probably in correspondence with the vegetable diet. 



We have seen (p. 491) that the body cavity or ccelom is de- 

 veloped as a split in the mesoblast sheets. The cells of the 

 splanchnic and somatic layers which face towards this form 



'o.st. 



Fig. 377. — Diagrams to illustrate 

 the formation of the central 

 nervous system of the frog. 



A, The folding off of the neural canal ; 

 B, transverse section of the fore-brain 

 at the hatching stage. 



c.c, Neural canal ; d.r., position in which 

 dorsal root arises; ec, epiblast ; ec'., 

 infolding of the epiblast of the neural 

 plate to form central nervous system ; 

 J-br.i fore-brain; Is., lens; op.c, 

 inner wall of optic cup, which will 

 form retina proper ; op.c'., outer wall, 

 which will form pigment layer ; o.st. y 

 stalk of optic vesicle. 



