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. 17(5. Fig. 177. Fig. 178. 



FlGS. 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 



f the yolk-sac. 



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



hirteenth 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 



