144 DEVELOPMENT OF THE FROG. 



spinal cord, and separated from each other by connective tissue 

 septa. Later on, the walls of the myotomes thicken consider- 

 ably, 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 myo- 

 tomes 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 r 

 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 midventral 

 line, the cavities of the two sides soon become continuous. The 

 anterior portion of the coslom is very early shut off from the 

 hinder part as the pericardial cavity. {Cf. Figs. 29 and 30.) 



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 

 free surfaces of both layers, i.e., the cells lining the body cavity, 

 become the peritoneum, from which, as we have already seen, the 

 ovaries and testes are formed. 



M. Development of the Skeleton. 

 1. The Vertebral Column. 



The earliest skeletal structure, and for a time the only one, 

 is the notochord, the development of which from the hypoblast 

 of the mid-dorsal wall of the mesenteron has already been 

 described. It forms a cellular rod extending from the blastopore 

 to the pituitary body ; and as the tail is formed, it extends back 

 into it. The notochord consist of vacuolated cells, filled with 

 fluid, and is invested by a delicate structureless sheath, 



Aboiit the time of appearance of the hind legs, a delicate 

 skeletal tube, at first soft but soon becoming cartilaginous, is 

 formed round the notochord from the mesoblast. This tube 

 grows upwards at the sides of the spinal cord, as a pair of longi- 



