THE THIRTEENTH AND FOURTEENTH DAYS. 477 



Embryos of the Thirteenth and Fourteenth Days, 



His' embryo, E. One of the youngest human ova containing 

 a distinct embryo was obtained by Professor His in 1869, and 

 was carefully described by him under the distinguishing letter 

 E. This embryo, which is at present deposited in the Anato- 

 mical Museum at Basle, is estimated to be about thirteen days 

 old : it is represented from the right side in Fig. 176 ; and in 

 diagrammatic sagittal section in Fig. 188. 



The entire vesicle (Fig. 188) is a thin-walled sac, measur- 

 ing 8*5 mm. by 5'5 mm. and covered all over with branched 

 villi. The contained embryo (Fig. 176) is 2-1 mm. long, and 

 is attached at its hinder end, by a short thick stalk, to the inner 

 surface of the vesicle. A slight constriction separates the 



FIG. 176. FIG. 177. FIG. 178. 



FIGS. 176, 177, 178. Outline figures, from the right side, of three Human 

 Embryos, estimated to be of the thirteenth or fourteenth days. (From 

 His.) x 5. 



FIG. 176. Embryo lettered by Professor His, E (cf. Fig. 188). 



FIG. 177. Embryo described by Allen Thomson. 



FIG. 178. Embryo lettered by Professor His, SR {of. Fig. 179). 



embryo ventrally from the yolk-sac, which measures 2'3 by 

 1*6 mm. Covering the embryo, but at a short distance from it, 

 is a membranous fold, which is clearly the inner or true amnion. 

 The embryo itself presents along its dorsal surface a shallow 

 neural groove, bounded by prominent neural folds; and the 

 only other organs visible on the surface are a pair of longi- 

 tudinal folds, formed by the two halves of the heart, and lying 

 between the anterior end of the embryo and the yolk-sac. 

 From the heart, vessels can be traced, running over the surface 

 of the yolk-sac. 



His' embryo, SR. This is a well-preserved embryo of the 

 thirteenth day, slightly older than the embryo E, but very 

 similar to it in all important respects. 



The entire vesicle measures 8 to 9 mm. in diameter, and 



