ON THE FATE OF THE MUSCLE-PLATE. 119 



same way in mammals, and is seen still more clearly in Rat 

 embryos at fifteen to seventeen days (fig. 16). Each root of 

 the nerves divides into two unequal branches — the dorsal root 

 beyond the ganglion, the ventral root directly. Of these 

 branches the smaller is superior, the larger inferior in both 

 cases. The larger branches unite to form the main trunk of 

 the nerve, or the inferior primary division ; the smaller 

 branches combine to form the superior primary division. This 

 is directed upwards and outwards, and subdivides as it passes 

 towards the surface. 



In Chicks at four days six hours, the condition of the nerves 

 in the trunk between the limbs is slightly more advanced, but 

 presents no change of any note. In the regions of the limbs, 

 however, the plexuses are now formed. In transverse sections 

 through the embryo, the nerves are found, on entering the 

 limbs, to divide into two fairly well-defined strands, separated 

 by a central mass of mesoblast, which is in active growth, and 

 preparing to form the cartilaginous basis of the limb. The 

 nerves, in fact, spread out around this central core, and arrange 

 themselves into two sets, one dorsal the other ventral. These 

 dorsal and ventral branches of the nerves only pass a short 

 distance into the limbs, and are not so well defined as in em 

 bryos a few hours older. But even now the process of plexus 

 formation may be seen. 



When longitudinal vertical (sagittal) sections are made con- 

 tinuously through the body, it is seen that the nerves to the 

 limbs, besides forming the dorsal and ventral branches above 

 mentioned, unite with adjacent nerves at the root of the limb 

 to form a well-defined plexus. In the Fowl three main trunks 

 form the brachial plexus, 1 — the first thoracic and the last two 

 cervical nerves, with in addition a small branch from the more 

 anterior cervical nerve. When sagittal sections are made, the 

 limbs being divided transversely at their roots (fig. 9, «), the 

 axillary artery (art.) and vein (v.), with the three main nerves 

 (N., 1, 2, 3), are divided just outside the body cavity (B. C), 

 and as they lie in the body wall (B. W.), before their entrance 

 1 Macartney, Art. " Birds," ' Rees's Cyclopaedia,' 



