DEVELOPMENT OF THE FROG. 551 



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- 

 vertebrse) above, and the unsegmented lateral plates below. 

 The lateral plates split into two layers, the splanchnic 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, external gills 

 grow out from the visceral arches, and the larva, still within 

 its gelatinous case, exhibits peculiar lashing movements of 

 the tail. Eventually, about a fortnight after the eggs are 

 laid, it escapes from the surrounding jelly and swims freely 

 in the water. At this time there is a cloacal opening, but 



the mouth has not yet 

 -ep. appeared. There is a 



large horse-shoe-shaped 

 sucker on the under 

 -arch, surface of the head, by 

 means of which the tad- 

 pole attaches itself to 

 foreign objects. 



In the next stage of 

 development, which ex- 

 tends from the time of 

 FIG. 186 -Gastrula stage of Newt. hatching until the com- 

 (After HERTWIG.) me ncement of the meta- 



te^'^^yoUc^fi'lfS^s^e^^OT^^tyi morphosis, many and 



important changes take 



place. The mouth, which has previously been merely a 

 blind pit, opens into the gut, the gut itself lengthens rapidly, 

 and becomes coiled like a watch spring ; the tadpoles feed 

 eagerly on vegetable matter and increase in size. The 

 mouth is " bordered by a pair of horny jaws, and fringed 

 with fleshy lips provided with horny papillae," the sucker 

 behind it becomes paired, and is gradually less used as the 

 power of locomotion increases. About the time when 

 the mouth is opened, four gill clefts open from the 

 pharynx to the exterior. The external gills shrivel, and are 

 replaced by an internal set ; these are enclosed on either 



