116 A. M. PATERSON. 



anything approaching the complexity of the adult limbs sooner than 

 the ninth day. 



III. Conclusions. 



1. On the fate of the muscle-plate and development of the muscles of the 

 limbs. 



In Elasmobranchs the muscles of the limbs are formed by the 

 muscle-plates which grow down, and as they pass the roots of the 

 limbs give off small buds, which become separated off and form the 

 starting point of the muscle formation in the limbs.* In higher 

 Vertebrates, while it has been held probablef that the muscle-plates 

 do not enter into the formation of these muscles, it has not been 

 shown satisfactorily how they do arise, and what becomes of the 

 muscle-plates, It is evident from the foregoing details that the 

 muscle-plates, in the regions of the limbs, stop short in their down- 

 ward growth, do not pass farther than the base of the limb, and are 

 not concerned in any way 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 disposi- 

 tion of the muscle-plates occurs. They grow down in the body wall, 

 and eventually become converted into the longitudinal 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 conver- 

 sion of the cells into longitudinal fibres, which merge with those of 

 the inner layer ; but this is not certain. In Elasmobranchs each 



* Balfour, "A Monograph on the Development of Elasmobranch Fishes," London, 

 1878. 



t Kolliker, " Entwickelungsgeschichte d. Menschen u. der hdheren Thiere," Leipzig, 

 1879. 



