ON THE FATE OF THE MUSCLE-PLATE. 117 



fusiform cell extends from end to end of the muscle-plate.* 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 develop- 

 ment of the limb muscles. They first appear as double layers of 

 dorsal and ventral cells, which layers are simple, without segmenta- 

 tion, and derived from the mesoblast cells of the primitive limb bud. 

 In Elasmobranchsf 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 down- 

 ward growth and a cutting off of part of certain muscle-plates occur; 

 the portions cut off undergo further growth, passing into the limb 

 bud, fusing together, and becoming differentiated into dorsal and 

 ventral strata. In birds and mammals the same end is reached 

 without these preliminary steps, and without the intervention of the 

 muscle-plates. The definite relations which these simple muscular 

 layers bear to the nerves of the limbs, throw light on the evolution of 

 the limb plexuses. Each nerve, passing into a particular region of the 

 limb bud, divides into dorsal and ventral branches, to supply the dorsal 

 and ventral surfaces respectively of that particular portion. As the 

 mesoblast forming the limb bud becomes more differentiated, so as 

 to give rise to the muscular layers, the portions opposite to and 

 originally derived from the same somites as the nerves become fused, 

 forming simple muscular layers in the first place. The nerves 

 therefore fuse together ; the dorsal branches forming a dorsal band, 

 and the ventral branches a ventral band, which pass out, and are 

 finally lost in these simple muscular layers. 



2. On the grovjth and development of the spinal nerves. 



It is difficult to demonstrate clearly, but it is next to impossible 

 to deny, that the spinal nerves are developed from epiblast through- 

 out their whole length. From the numerous sections which I have 

 examined at different periods of growth, I have traced the spinal 

 nerves, not only the nerve-roots, but also the trunks and the 

 plexuses, as a centrifugal growth from the spinal cord. The growth 



* Balfour, " Comparative Embryology," p. 552. 

 I Balfour, "Monograph on Elasmobranchs." 



