124 A. M. PATEESON. 



with the production of the limb muscles. These are formed 

 by the differentiation of the mesoblastic cells forming the 

 primitive limb buds. These cells form, in the first place, a 

 central cartilaginous bar, above and below which, in the second 

 place, are developed a double dorsal and a double ventral layer 

 of simple muscle, which later on becomes more complex in 

 its arrangement and forms the muscles of the adult. 



In the region of the trunk, between the limbs, a different 

 disposition of the muscle-plates occurs. They grow down in 

 the body wall and eventually become converted into the longi- 

 tudinal muscles of the trunk. They do not, however, appear 

 to assist in the formation of the sub-vertebral (hyposkeletal) 

 muscles. 



In both regions the growth and differentiation of the parts 

 of the muscle-plates are alike. The outer layer disappears 

 gradually ; the inner layer of the plate being converted into 

 the longitudinal fibres. The disappearance of the outer layer 

 is possibly due to the conversion of the cells into longitudinal 

 fibres, which merge with those of the inner layer ; but this is 

 not certain. In Elasmobranchs each fusiform cell extends 

 from end to end of the muscle-plate. 1 In birds and mammals 

 this is not so. Each fibre is considerably shorter than the 

 breadth of the somite. 



The chief point of interest here is in connection with the 

 development of the limb muscles. They first appear as double 

 layers of dorsal and ventral cells, which layers are simple, 

 without segmentation, and derived from the mesoblast cells of 

 the primitive limb bud. In Elasmobranchs 2 this stage in the 

 development of the limb muscles is a secondary one, and is 

 preceded by events which are omitted in higher Vertebrates. 

 The process of evolution of the limbs in birds and mammals is 

 therefore shortened. In Elasmobranchs a downward growth 

 and a cutting off of part of certain muscle-plates occurs ; the 

 portions cut off undergo further growth, passing into the limb 

 bud, fusing together, and becoming differentiated into dorsal 



1 Balfour, ' Comparative Embryology,' p. 552. 



2 Balfour, ' Monograph on Elasmobranchs.' 



