THE SYMPATHETIC NERVOUS SYSTEM IN MAMMALS. 
1G5 
out slightly before their termination. The nerves consist for the most part of fibres : 
the cellular elements are plainly mesoblastic, and help in forming the nerve-sheaths. 
Tliere is no sign as yet of the formation of the superior primary division, or 
splanchnic branch of the nerve. The ventral root passes out entirely below the 
dorsal ganglion, which is easily defined. The interval before mentioned between tlie 
aorta, cardinal vein, spinal nerve, and spinal cord, is now filled up by a mass of 
mesoblastic cells (intermediate cell-mass) in process of vertebra formation. The cells 
are large, often with several nuclei, and are in an active state of division. They all 
stain equally with borax-carmine, except the nuclei of blood corpuscles, which stain 
more deeply. 
Sagittal sections of embryos from the same uterus show the spinal nerves in 
transverse section; but there is no evidence of the existence of the sympathetic cord 
or ganglia. 
In Mouse embryos of about 8 days, and in Rat embryos of a similar age (the axial 
length of the latter being 7 mm.), the first indication of the formation of the sympa¬ 
thetic cord has been found. The general condition of development in both these 
animals corresponds with that of the Chick at the end of the third and beginning of 
the fourth day. The spinal cord is cellular, a narrow belt of fibres being present on 
each side in the neighbourhood of and between tlie nerve-roots. The spinal nerve 
has extended almost as far as the somato-splanchnic angle, and to the cardinal vein in 
places where the latter is present; and in some sections the superior primary division 
is apparent. The pulmonary diverticulum from the alimentary canal has become 
divided into the two main bronchi. The Wolffian duct and Wolffian body are 
distinct. 
The Mouse embryos (Plate 22, fig. 3) were hardened in picric acid and stained with 
borax-carmine. A change is now apparent among the cells of the mesoblast surrounding 
the aorta (Ao.). Li the interval between the latter and the cardinal vein (C. V.), instead 
of a uniformity in the cells composing the intermediate cell-mass, an irregular group of 
cells is seen among them {Sy.), lying on the ventral side of the intercostal arteries, and 
in close proximity to the aorta. On close examination, this mass of cells is strikingly 
different from the surrounding mesoblast. The cells stain more deeply ; the nuclei are 
larger, and often possess a considerable number of nucleoli ; and the cells form a 
rounded clump closely massed together, only in some sections clearly demarcated from 
the cells in which they are imbedded. This mass is bilaterally symmetrical, and is 
evident in continuous series of sections from the level of the cephalic border of the 
fore-limb anteriorly, where it ends abruptly, to the middle of the trunk—the region of 
the stomach—posteriorly. It is comparatively large anteriorly, and gradually tapers 
off and becomes indistinct posteriorly. The mass in front is double the thickness 
of that seen further back. There is no connexion, fibrous or cellular, with the spinal 
nerves or ganglia. 
The Rat embryos of the same age were hardened in spirit, and stained with aniline 
