TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 283 



if they are cut, may be easily recognized in sections. The development of the 

 trabeculae corresponds to the formation of blood sinusoids of the heart. The 

 trabeculae consist of young muscle-cells, and each bundle of cells is closely invested 

 by the endothelium of the heart. The blood thus circulates freely between the 

 trabeculae, but remains, as in every blood-channel, separated by the endothelium 

 from the neighboring tissue. The tissues of the heart are thus enabled to get 

 their nourishment from the blood circulating through the organ. The sinusoidal 

 type of circulation which we here encounter appears in the heart of all vertebrate 

 embryos, and is the permanent form of circulation in the frog. In mammals, on 

 the other hand, although the sinusoidal circulation is kept throughout life and 

 the ventricles always have their trabecular structure, yet we find, in addition, the 

 development of a true capillary circulation to supplement the sinusoidal. This 

 capillary circulation is supplied by the coronary arteries, and develops compara- 

 tively late. Between the auricles and the ventricles the heart is narrow. This 

 constricted region is known as the auricular canal. A broad partition, S.a.c, 

 divides the cavity of the auricular canal into right and left channels, forming open 

 vessels between the auricles and ventricles. From the lower edges of these channels 

 flaps of tissue project into the ventricles. The flaps are the anlages of the atrio- 

 ventricular valves. 



Sections through the Anterior Limbs to Show the Brachial Plexus (Fig. 197). 

 Figure 197 was drawn from a single section, except that the nerves in the limbs 

 represent a reconstruction from several adjacent sections. The limb-bud, A.L, 

 projects freely from the side of the body, is covered by ectoderm, EC, and filled 

 with a very dense tissue, the cells of which show no very clear histological dif- 

 ferentiation. The spinal cord, Sp.c, is fairly well advanced in its development 

 at this level, and shows a darker, inner layer, Epen, a middle gray layer, cm, and 

 an outer neuroglia, Ec.gl. The cord is completely surrounded by the developing 

 pia mater, which is quite thin, but highly vascular. The ganglia are cut almost 

 symmetrically on the two sides and show their dorsal roots. The descending 

 trunk from each ganglion is joined by the ventral roots, V.R, which arise from 

 the ventral zone of the cord in several bundles which unite about the same time 

 with both one another and the dorsal root to form the main nerve-trunk, N.S, 

 which enters into the formation of the brachial plexus. Just after the junction 

 of the two roots the nerve gives off a branch which runs obliquely dorsalward 

 into the anlage of the dorsal muscles, Muse. This branch is, of course, the dorsal 

 ramus. The trunk, N, which runs toward the limb is the ventral ramus. Below 

 the spinal cord is the notochord, Nch, which is completely surrounded by a dense 

 mass of mesenchymal cells, Vert, the anlage of an intervertebral disc. Triy^ 

 in the section are the two descending aortas, Ao, which are "t 

 uniting to form the single median dorsal aort"*. On either si< ? * 

 of darker cells from the aorta upward toward the late r ' 

 vertebral disc. The dark cells belong to the sympathc' 



