48 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



spheres (ch) is better shown than in the preceding figure, as is also 

 the general form of the optic cup (oc). On the left the nasal cavity 

 (//) is seen as an elongated slit with thick walls; it is cut near, but 

 not through, its opening to the exterior. The same gill cleft (g) 

 that was seen in the preceding figure is seen here as a narrow, trans- 

 verse cleft, open at both ends. Between the notochord (nt) and the 

 spinal cord (sc) is the same, though now double, blood-filled vessel 

 (bv) that was seen in the preceding section. The other blood-ves- 

 sels are larger here than in the more anterior region. There is a 

 faint condensation of mesoblast in the neighborhood of the noto- 

 chord, and a more marked condensation (nip) farther toward each 

 side is the curiously shaped muscle plate. 



Figure 17c is through the heart region, and that organ is cut 

 through the opening from the lower or ventricular into the upper or 

 auricular chamber. The thickening of the wall of the ventricle, 

 which was noticed in the preceding stage, has increased to such 

 an extent that there is now a marked difference in the thickness of 

 the ventricular and auricular walls. As in the preceding stage, the 

 body wall is torn, probably in handling, so that it appears to be 

 incomplete around the ventral side of the heart. Dorsal to the heart 

 two small circular holes (cut) with thick walls are the cesophagus 

 and trachea, cut anterior to the point of bifurcation of the latter into 

 the bronchial or lung rudiments. On either side of these struc- 

 tures is an elongated blood-vessel (etc), the anterior cardinal vein, 

 its elongation being due to the fact that it is cut at the place where 

 it turns downward to empty into the heart. Dorsal to the cesoph- 

 agus are the aorta; (ao), which are here cut just at the point where 

 the two vessels unite to form one ; the next section, posterior to the 

 one under discussion, shows an unpaired aorta. The notochord (nt) 

 and spinal cord (sc) need no description, except to note that the 

 latter shows active histological differentiation, numerous mitotic 

 figures being seen under higher magnification, especially in the 

 cells that line the spinal canal. On the right of the cord the edge 

 of a spinal ganglion (sg) is seen, in connection with which in other 

 sections are seen the clearly defined nerve roots. The condensation 

 of mesoblast around the notochord is quite evident, and in close 

 contact with this medial condensation are two very characteristic, 

 S-shaped muscle plates (mp), which extend from the level of the 

 dorsal side of the spinal cord to the upper limits of the cardinal 

 veins. In some sections the muscle plates even yet show slight 

 remains of the myocoel at the dorsal end. 



Figure ijd is in the region of the posterior end of the heart (ht), 

 which is cut through the tip of the ventricle, and the anterior end of 



