ON THE FATE OP THE MUSCLE-PLATE. 121 



body cavity. The smaller nerve represents the lateral branch 

 of the adult, and is directed downwards and outwards through 

 the muscle-plate, on the outer side of which it divides and is 

 finally lost. The cord to the sympathetic nerve can be followed 

 farther than before ; but I have been unable to trace its con- 

 nections with the roots and trunk of the spinal nerves. 



In the limbs the early changes occurring in the blastema, 

 which lead to the production of the osseous and muscular 

 systems, have already been described. The nerves are of large 

 size, and can be traced more than halfway through the limb 

 towards the distal end. At the root of the limb the main 

 trunk (fig. 12, N.) can be seen in transverse sections to divide 

 into two well-defined trunks, which are clearly homologous 

 with the terminal branches of the nerve in the region of the 

 trunk at this date (fig. 11). These two large nerves are re- 

 spectively dorsal (d) and ventral (v) ; and enclose between 

 them the densest portion of the blastema (3). This enclosed 

 portion has already been described as consisting of three parts 

 — a central part, which is going to form cartilage, and a dorsal 

 and ventral part, the elements of muscular tissue. On the 

 dorsal surface of the dorsal nerve, and on the ventral surface 

 of the ventral nerve, are other layers of mesoblast cells (1 and 

 2) undergoing division preparatory to the production of muscles. 

 In longitudinal sections the three main nerves supplying the 

 fore limb can be traced as before in successive sections, each 

 dividing into dorsal and ventral branches ; and these again 

 uniting with adjacent nerves to form two flattened bands, 

 which pass to the dorsal and ventral surfaces of the limb. 



The nerve-trunks are almost entirely fibrous now, with rows 

 of deeply-stained nuclei arranged alone and among the wavy 

 fibres. These are evidently the connective-tissue elements of 

 the nerve-trunks. Towards their terminations the fibres are 

 fewer and the fusiform nerve-cells more abundant. 



In Rat embryos, between twelve and fourteen days old, 

 exactly the same condition of development of the nerves is 

 found as in the Chick at four days twelve hours. The state of 

 development of the body generally is the same, the limbs exist 



