172 OUTLINES OF CHORDATE DEVELOPMENT 



of the myotome the myoccel is early obliterated. The 

 myotomal cells or muscle cells, elongate antero-posteriorly 

 through the entire segment; the formation of muscle fibrillsB 

 in these cells begins very early (5 mm.) on the side toward 

 the chorda (Fig. 53). 



From the ventro-medial portion of the myotome, cells prolif- 

 erate and move downward below the chorda, and upward be- 

 tween the chorda and the myotome, forming the rudiment of 

 the sclerotome. The somite now separates entirely from the 

 lateral plate, and soon the sclerotome separates from the somite, 

 and extends dorsally around the nerve cord, forming a consider- 

 able mesenchymal mass surrounding this and the notochord. 

 This is the region where the cartilaginous vertebral column 

 forms later. 



Just after the separation of the sclerotome (5 mm.) the 

 myotome and dermatome send down a ventro-lateral out- 

 growth, which soon separates from the myotome and forms 

 later the ventral musculature (Fig. 60); from the myotomes 

 of the limb regions these outgrowths extend into the rudiments 

 of the limbs, later giving rise to their voluntary musculature. 



The cutis plate breaks into groups of branched mesenchyme 

 cells, some of which become applied to the inner surface of 

 the ectoderm and form the dermal layer of the dorsal half of 

 the embryo, while others pass in between the mytomes, 

 forming the connective-tissue septa or myocommata. 



In the trunk region of the frog, thirteen pairs of somites are 

 formed altogether, but the two anterior pairs disappear about 

 the time the limbs appear, leaving eleven in the adult. The 

 region of these two transitory somites later becomes incorpo- 

 rated into the head, as the occipital region. The accompanying 

 table, based upon the observations of Elliot, summarizes the 

 history of the somites and spinal nerves of the body region 

 of the embryo. In the tadpole there is, of course, a large, and 

 varying, number of somites in the tail region; Harrison has 

 counted about forty-five pairs in a 5.5 mm. larva of Rana 

 virescens. All posterior to the thirteenth (eleventh of the 

 adult series) disappear during metamorphosis. 





