DEVELOPMENT OF THE FROG. 



601 



Internally, a differentiation 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 hypo- 

 blast. Each of these divides into 

 the primitive segments (proto- 

 vertebrae) above, and the un- 

 segmented 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 dis- 

 tinctly divided into regions, the 

 eyes bud out from the brain, a 

 rudiment of the gills appears, 

 and the larva, still within its 

 gelatinous case, exhibits peculiar 

 lashing movements of the tail. 



Eventually, about a fortnight 

 after the eggs are laid, the larva 

 escapes from the surrounding 

 jelly and swims in the water. At 

 this stage and for some time the 

 ectoderm is ciliated. There is a 

 cloacal opening, but the mouth is 

 not yet more than a dimple. A 

 glandular crescent, often mis- 

 named a sucker, lies on the 

 under surface of the head, and 

 secretes a sticky slime, by means 







FIG. 326. Dissection of 



tadpole. After Milnes 

 Marshall and Bles. 

 , . , , -, i Z>L., Lower lip ; If., ventricle of 



of which the tadpole attaches heart; DE., oesophagus; JVA., 



itc^lf 1-r fnrpicrn rHiprt<5 Thf head kidney; A., aorta; J?., 

 ODjeClS. ine kid . jf ureter; DO., 



protruding gills soon become 

 branched. There are three of 

 them on each side, the first the 

 largest. They are covered with 

 ectoderm, and are borne on 



kidney ; KU., ureter ; DO., 

 cloaca; LH., hind-limb; KV., 

 opening of ureter into cloaca; 

 GR.) genital ridge; GF. t fatty 

 body ; LF., fore-limb ; OG., gills ; 

 a t epidermis ; b t dermis. 



the outside of the first 



