440 A Three Weeks' Human P^niliryo 



Especially noticeable in this figure are: The short striatal folds; the 

 apparent continuity of the albicantial fold with the roof fold (cf. Figs. 8, 9); 

 the cerebellar folds 1 and 2 (cf. Fig. 11, cbl.) ; the cleft in the oblongata fold 

 4, on either side of which arise the roots of the Vllth and Vlllth nerves 

 (shown by dotted circles) ; the cut ends of the ring of mesoderm surround- 

 ing the neuropore; the intrusion of mesoderm into the cephalic flexure; the 

 fact that no mesoderm intervenes between hypophysis and infundibulum on 

 the middle line; the union of all layers in the roof of the oblongata; the dark 

 areas in the pharynx, indicating with their dotted extension the membranous 

 parts of the 1st to the 4th gill pouches; the notochord touching the caudal 

 wall of the hypophysis and coming in close contact with the roof of the 

 pharynx between the level of the 1st and 3d gill pouches. 



Plate III. 



Fig. 5. A ventral view of a segment of the model (Fig. 1) extending from 

 section 247 to 155. 



The left side of the head is dissected away to show: The relation of the 

 visual lobe, the neuropore, the olfactory and cerebral regions, and the roof 

 of the diencephal. 



The right side of the head shows: The thickened epithelium of the neuro- 

 pore; the olfactory region and its extension over the cerebrum; the future 

 lens; the mandible and gill-cleft-like pocket at the corner of the mouth. 



The caudal portion of the figure (Sec. 247) shows the 23d myotome appar- 

 ently continuous with the mesoderm of the leg-bud; the division of the aorta 

 into the umbilical branches, the left one looping over the caudal end of the 

 ccelom; the left umbilical vein with a branch from the leg; the right plexi- 

 form umbilical vein passing along the body-wall toward the heart. 



The middle part shows: The wide umbilicus turning to the right and 

 containing the thick-walled vitelline sac with its veins and arteries and its 

 union with the caudal intestine inclosed in mesentery; the reappearance of 

 the intestine in section near the union of the Wolffian duct and allantoic 

 stalk (cf. Fig. 17). 



Fig. 6. A ventral view of a deeper segment of the model. It extends from 

 section 185 to 155. In the head region (cf. Figs. 2-5) it is cut through; the 

 neuropore; the eye vesicles, partially constricted off from the stalk; the 

 albicantial folds (cf. Fig. 9); and the cephalic end of the thick-walled mesen- 

 cephal as it dips into the albicantial region. At N. Ill is a strand of meso- 

 dermic tissue but apparently no true nerve fibers. 



The caudal part of the figure (Sec. 185) shows: On one side, the appear- 

 ance of a myotome cut through the middle; on the other side the over- 

 lapping ends of two myotomes; the 8th mesonephric tubule opening into the 

 Wolffian duct and with its cap of thickened peritoneal epithelium (cf. Fig. 

 17) and its artery and vein. 



In the middle portion are: The umbilical veins at either side, the left show- 

 ing the greatly divided sinuses; the vitelline veins as they approach on either 

 side of the alimentary canal; the vitelline artery; the liver near its caudal 

 part embedded in the transverse septum and near the level where the bile 

 duct unites with the duodenum; the umbilicus turning to the right of the 

 specimen. 



