THE THIRTEENTH AND FOUKTEENTH 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. 1 77. 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 (of. Fig. 188). 



FIG. 177. Embryo described by Allen Thomson. 



FIG. 178. Embryo lettered by Professor His, SR (cf. 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 



