DEVELOPMENT OF THE FROG. 



gut prevents the body acquirin: 

 shape. The blastopore as usual 

 marks the posterior region of the 

 body. 



The processes which follow are 

 already in outline familiar to the 

 student. Along the mid-dorsal 

 line an epiblastic neural plate is 

 differentiated. The edges be- 

 come raised into the neural 

 folds ; these approach one 

 another, and, fusing together, 

 form the medullary or neural 

 canal. At the posterior end this 

 communicates with the archenter- 

 on for a time by the neurenteric 

 canal. Internally, a differentia- 

 tion of hypoblast forms the 

 notochord along the mid-dorsal 

 line of the archenteron. At 

 each side of this lie masses of 

 mesoblast which have been split 

 off from the hypoblast. Each 

 of these divides into the primitive 

 segments (proto-vertebrae) above, 

 and the unsegmented lateral 

 plates below. The lateral plates 

 split into two layers, the splanch- 

 nic or inner investing the gut, 

 the somatic or outer layer being 

 applied to the epiblast ; the 

 space between the two layers is 

 the body cavity. The body now 

 becomes distinctly divided into 

 regions, the eyes bud out from 

 the brain, a rudiment of the 

 external gills appears, and the 

 larva, still within its gelatinous 

 case, exhibits peculiar lashing 

 movements of the tail. 



Eventually, about a fortnight 



at once the fish-like 



-DL 



FIG. 285. Dissection of tad- 

 pole. After Milnes Mar- 

 shall and Bles. 



DL., lower lip; //., ventricle of 

 heart; DE., oesophagus; NA., 

 head kidney; A., aorta; fC., 

 kidney; A" 6*., ureter; Z)O., 

 cloaca; LH., hind-limb; AT'., 

 opening of ureter into cloaca ; 

 GJ?., genital ridge; GF., fatty 

 body ; Lf., fore-limb; OG. , in- 

 ternal gills ; a, epidermis ; /', 

 dermis. 



after the eggs are laid, 



