ORGANS OF LARVA 64I 



body starts as a median longitudinal groove on 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 tad- 

 pole, 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. 



MESOBLASTIC TISSUES 



We have seen (p. 634) that the body cavity or coelom is 

 developed as a split in the mesoderm sheets. Those cells of the 

 splanchnic and somatic layers which face towards this form the 

 coelomic epithelium. The greater part of the coelom becomes the 

 abdominal (pleuroperitoneal) cavity, surrounding the gut on all 

 sides except in the mid-dorsal line, where the mesentery is left. 

 A forward ventral prolongation of the coelom becomes the peri- 

 cardial cavity. The muscles of the gut are formed from the 

 splanchnic layer, the body muscles from the myotomes, which, 

 with sclerotomes (p. 564), have arisen from the epimeres (' meso- 

 blastic somites '), though they are displaced in the adult. The 

 skeleton arises from mesenchyme, mainly from sclerotomes. 



BLOOD VESSELS 



The heart appears some time before hatching. It is at first a 

 straight tube, which arises below the pharynx. Subsequently 

 the tube is thrown into an S shape (see p. 595) and becomes divided 

 by partitions into its several chambers. The endothelium or 

 pavement epithelium which lines the heart arises by the rearrange- 

 ment of some scattered mesenchyme cells between the splanchnic 

 layer of mesoderm and the ventral endoderm of the gut, and the 

 muscular tissue is formed by a folding of the splanchnic layer 

 itself (Fig. 499). The space between the splanchnopleure and 

 somatopleure in the region of the heart forms the pericardial 

 cavity. At this time it is continuous with the rest of the coelom : 

 later, communication between the pericardial and abdominal 

 cavities is abolished by the formation of the great veins. The veins 

 and arteries arise, by separation of cells of the mesoderm, as 

 irregular spaces which join up to form blood vessels. The earliest 

 vessels to appear are the vitelline veins, one on each side of the 

 yolk mass in the splanchnic mesoderm. These join the sinus 



