150 



DEVELOPMENT OF THE FROG'S EGG [Cii. XIV 



The development of the musculature of the head, limbs, and 

 ventral body-wall takes place at a later stage. A description 

 of the origin and development of these structures is beyond 

 the limit of the present account. 



The Heart and Blood-vessels 



The heart appears at the time when the medullary folds have 

 rolled in, and have met along the mid-dorsal line ; it lies below 

 the pharynx, and anterior to the liver (Fig. 37, B). The meso- 

 derm in this region shows a tendency to split into two sheets 

 and, where the heart is about to develop, a cavity, a part of 



P H 



Fig. 45. -Three stages in development of heart. E. Endothelium. PE. Pericar- 

 dium. PH. Pharynx. W. Wall of heart. 



the coelom, appears between the sheets. A cross-section of the 

 larva (Fig. 45, A) shows on each side of the mid- ventral line 

 in the region of the heart the somatic and splanchnic layers 

 widely separated from each other. The coelomic cavities of 

 the right and left sides are not continuous across the middle 

 line, but anterior and posterior to this section the crelomic 

 cavity is found to be continuous before and behind with the 

 general crelomic space on each side. A few scattered cells 

 lie in the middle line between the splanchnic layer and the 

 ventral wall of the pharynx (Fig. 45, A). These cells have 



