136 DEVELOPMENT OF THE FROG 



loses its connection with the aorta, but persists as the pulmo- 

 cutaneous arch of the adult, from which both pulmonary and 

 cutaneous arteries arise.* 



L. Development of the Muscular System and the Coelom. 



The splitting of the mesoblast into outer or somatopleuric, 

 and inner or splanchnopleuric layers has already been described. 

 (Gf. Fig. 26, p. 116.) 



In the body the mesoblast becomes very early divided on each 

 side into (1) a vertebral plate, which is dorsally situated, and 

 lies alongside of the spinal cord and notochord; and (2) a 

 lateral plate, which surrounds the side of the body. 



The vertebral plate very early becomes divided transversely 

 into muscle-segments or myotomes, which form a row of hoUow 

 and somewhat cubical bodies, lying along each side of the spinal 

 cord, and separated from each other by connective tissue septa. 

 Later on, the walls of the myotomes thicken considerably, 

 especially the inner walls, and become converted very largely 

 into muscles ; while the cavities become obliterated. 



The myotomes may be well seen in the tail of the tadpole, 

 where they form the great lateral sheets of muscle on each side 

 of the tail, by which the swimming movements are effected. 

 Owing to the transparency of the tail, their arrangement can 

 be very readily made out ; the septa dividing the successive 

 myotomes from each other are not transverse, but > shaped, 

 with the angles directed forward towards the head. 



The lateral plates are also in part converted into muscle ; 

 the two layers, somatopleuric and splanchnopleuric, remain 

 comparatively thin, but the space between them widens out 

 considerably, and becomes the body cavity or coelom. This at 

 first consists of two separate halves, right and left ; but, owing 

 to the splitting of the mesoblast extending down to the mid- 

 ventral line, the cavities of the two sides soon became con- 

 tinuous. The anterior portion of the coelom is very early shut 

 off from ,the hinder part as the pericardial cavity. (Cf. Figs. 

 28 and 29.) 



The outer or somatopleuric layer of mesoblast, with the epi- 

 blast, forms the body-wall of the adult ; the inner or splanchno- 

 pleuric layer, with the hypoblast, forms the wall of the 

 alimentary canal and its diverticula. The cells covering the 

 * See note, p. 135. 



