EMBRYO OF 20 MM. 247 



pulmonary artery, A . pul. The two arteries join a little nearer the head and on 

 the left side of the embryo to form a single trunk, the main pulmonary artery. 

 Originally the pulmonary arteries arise symmetrically as branches from the fifth 

 aortic arch. They soon unite, however, throughout the greater part of their 

 extent, forming a single vessel. The two arteries shown in our figure represent 

 the two original symmetrical vessels where they are about to enter the lungs. 

 On the ventral side of the section various cardiac structures are shown, but so cut 

 that the picture is not very instructive. It will suffice to refer to the explanation 

 of the figure for the identification of the parts. 



Section through the Posterior Limbs (Fig. 143). — Although this section is from 

 a transverse series, yet, owing to the curvature of the body, it shows the spinal 

 cord cut very obliquely. The three layers of the cord, the ependymal, epen, the 

 cinerea or neurone layer, Cin, and the ectoglia are well marked. Something of 

 the dorsal root, D. R, and of the ganglia, G, of a lumbar nerve are also shown in 

 the section. The nerves have already joined together to form a very complex 

 lumbar plexus, sections of portions of which appear at various points. These are 

 all indicated by the reference letter A^ in the figure, it being thought not desirable 

 to attempt an identification of each component of the plexus. The plexus is more 

 or less symmetrically placed on the right and left, at about the level of the intes- 

 tine, Rect. The limbs are large projections extending downward and containing 

 in their interior the cartilaginous anlages, carf, cart", of the skeleton of the limb ; 

 and, around these, darker masses of tissue, the developing muscle-fibers. At the 

 lower edge of each limb is a blood-vessel, V. p, the so-called border or peripheral 

 vein, which extends completely around the edge of the developing hand and foot. 

 When the digits are developed, this vein becomes broken up, and out of its divi- 

 sions are formed the digital vessels. The section also passes through the penis. 

 Pen, in the center of which is the urethra, Ur. It shows here as a narrow epi- 

 thelial band entirely without any cavity, except a very small one at its external 

 dorsal end. The band is lighter in the center, owing to the fact that the nuclei 

 are grouped chiefly close to the two surfaces of the band. At the base of the limb 

 is situated the irregularly shaped section of the iliac vein, V. il. In the median 

 line may be noted the following structures. Immediately underneath the ner- 

 vous system is the arteria sulci, A. sul. The vertebra, Vert, and notochord, 

 Nch, resemble corresponding structures in the section last described, except that 

 their cytomorphosis is slightly less advanced. Below the vertebra lie the paired 

 anlages of the sympathetic nervous system, Sym, between which is the small 

 median caudal artery. The intestine, Rect, has its transverse diameter some- 

 what increased, so that it appears oval in the section. Around it is beginning the 

 differentiation of the mucosa muscularis. 



