466 



HUMAN ANATOMY. 



have become fibrillar throughout their entire thickness ; those of the lower ex- 

 tremity acquire a similar condition a month later. With the deeper extension of 

 the fibrillae the characteristic cross-striation appears, the nuclei migrating to the 

 periphery of the fibre as the less differentiated cytoplasm becomes invaded. The 

 sarcolemma appears by the time the entire fibre has become fibrillar. The sarcoplasm 

 surrounding the nuclei of the mature fibre represents the remains of the less highly 

 differentiated cytoplasm of the original muscle-cell ; that, however, separating the 



muscle-columns must be regarded 

 Fig. 489. as the product of a secondary dif- 



ferentiation. 



The designation ' ' cutis-plate, ' ' 

 applied to the compact outer epi- 

 thelioid portion of the myotome, 

 expresses the relation to the in- 

 tegument which has been widely 

 accepted, since this part of the 

 myotome is generally regarded as 

 concerned in the formation of the 

 connective-tissue portion of the 

 skin. This fate of the ' ' cutis- 

 plate" was long age denied by 

 Balfour, who held that both layers 

 of the myotome are concerned in 

 the formation of muscular tissue. 

 Kaestner ^ arrived at similar conclusions, and more recently Bardeen ^ has shown that 

 in the pig practically the entire epithelial lamella is converted into muscle. According 

 to this investigator, while some of the epithelial elements of the skin-plate degenerate, 

 the greater number undergo mitosis and give rise to myoblasts which, in turn, become 

 the spindle-cells from which the muscle-fibres are developed. The outer margin of 

 the epithelial lamella is sharply defined by a limiting membrane formed by the adja- 

 cent cells ; a somewhat similar but less pronounced boundary guards the inner con- 

 tour of the lamella. The external limiting membrane persists until the conversion of 

 the epithelioid elements into myoblasts and spindle-cells has been well established, by 

 which time the mesoblastic tissue surrounding the myotomes has grown in between 

 the latter and the adjacent ectoblast ; it is from this source, therefore, and not from 

 the " cutis-plate," that the connective-tissue layer of the integument is derived. 



The masses of embryonal muscle, or myomeres, derived from the somites are 

 early separated by the ingrowth of intersegmental septa of connective tissue which 



Developing voluntary muscle ; the fibres are stil! unstriated. X 525. 



Fig. 490. 



Developing muscle-fibres in which striation is just appearing. X 375. 



later support the intersegmental blood-vessels and nerves and, in the thoracic region, 

 the costal elements, and, by the ingrowth of a connective-tissue partition, each one 

 is further divided into a dorsal and a ventral portion, from which, in a general way, 

 the muscles associated with the spine and the antero-lateral body-walls are derived 

 respectively. 



In this primitive condition the trunk musculature is represented by a series of 



' Archiv fiir Anat. u. Phys., .Suppl. Bd., 1890. 

 ^ Johns Hopkins Hospital Reports, vol ix., 1900. 



