442 A Three Weeks' Human Embryo 



On the left, dissections clown to Sees. 99, 103, 110, and 128 show: The 

 relations of the gill arches, and the four gill pouches to the pharynx, the 

 larynx, esophagus; the ca?lomic cavities separated by the mesentery and only 

 partially divided into pericardial and abdominal regions by the lateral 

 infolding formed by the ducts of Cuvier; at the left the dissected cardinal 

 vein arching over the coelom and uniting with the jugular vein to form the 

 duct of Cuvier and thence dipping ventrad to join the sinus venosus (cf. 

 Figs. 10, 2); the right and left aortse near their point of union; the right 

 arm-bud with its thickened epithelium and the branches of the terminal 

 blood-vessels; the 10th myotome merging into the mesoderm of the left arm- 

 bud near its dorsal portion; well developed motor nerve roots. 



Fig. 12. A ventral view of the heart showing the somewhat greater 

 length of the right part of the common auricular chamber. The figure is 

 labeled as though the right and left sides were separate. By comparison 

 with Figs. 2-6, the relations are seen of the bulbus arteriosus and the sinus 

 venosus to the single tube forming the heart. 



Plate V. 



Fig. 13. A segment of the model (Fig. 1) from Sees. 5 to 20, showing: 

 The total foldings obi. 7, 8, 9 of the neural tube dorsad of the roots of the 

 Xth, Xlth, and 1st cervical nerves (cf. Figs. 3-4); the foldings of the skin 

 corresponding with those of the neural tube; the metatela rapidly widening 

 from the cephalic end of the cut surface (cf. Fig. 14); the close connection 

 of neural and epidermic epithelium. 



Fig. 14. From a photograph (X 471/2) of Sec. 25 of the human embryo 

 148 (cf. Figs. 1, 2, 4). It shows: The neural tube just at the ventral border 

 of the folds, obi. 8, 9, represented in Fig. 13; the cephalic, 1st, root of N. XI, 

 attached to the base of fold, obi. 8; the 2d root of N. XI, attached to obi. 9; 

 the Xlth N. as it passes through the 1st cervical and Froriep's ganglia; the 

 intimate union of the 2d and 3d cervical ganglia. 



The cilia lining the tube appear faintly and stop short of the dorsal margin 

 of the neural tube. 



At the right, the first four myotomes are seen, the 4th showing especially 

 well the dorsal division into two separate horns. Noticeable is a continua- 

 tion cephalad of similar cell-groups and epidermal corrugations representing 

 remnants of still more cephalic, occipital myotomes. 



Fig. 15. A photograph (X 471/2) of part of Sec. 44 (cf. Figs. 3, 4). showing 

 the neural tube wuth its cilia, metatela, and the relations of the Xth, Xlth, 

 and Xllth nerves. 



At the left of Fig. 14, the fold, obi. 7, is seen. At the right, it has dis- 

 appeared, to reappear as a more marked depression at the lower level of 

 Fig. 15, where the attachment of the Xth N. is found. 



At the left is seen the appearance of the 1st, 2d, and 3d myotomes at a 

 lower level than in Fig. 14, and at the right, at a still lower level, as they 

 recede farther from the skin and where the nuclei, related to the developing 

 muscle fibers, form a band across the myotome. At the left is a continua- 

 tion cephalad of the same segmented appearance of the epidermis as that 

 which lies over the myotome. 



