THE THIRD WEEK. 



487 



Coste's embryo (Fig. 196). An embryo described and figured by 

 Coste, about the precise age of which there is some doubt, appears 

 to belong to the commencement of the third week. The whole 

 vesicle measures 16'2 mm. along its greater diameter, and is 

 covered externally with short, slightly branched villi. The em- 

 bryo is attached to the inner surface of the vesicle by a short 



Fig. 189. 



Fig. 190. 



Fig. 191. 



Fig. 192. 





Fig. 193. 



Fig. 194. 



Fig. 195. 



Figs. 189 to 195. — Outline figures of seven Human Embryos of the third week. 

 (From His.) x 5. 



Fig. 189. — Embryo lettered by Professor His, Lg, and estimated as fifteen 



days old. {Cf. Fig. 197, p. 489.) 

 Fig. 190. — Embryo lettered by Professor His, Sch, and estimated as fifteen 



days old. 

 Fig. 191.— Embryo lettered by Professor His, M, and estimated as eighteen 



days old. 

 Fig. 192. — Embryo figured by Professor Allen Thomson, and probably about 



eighteen days old. 

 Fig. 193. — Embryo lettered by Professor His, BB, and estimated as about 



eighteen days old. 

 Fig. 194. — Embryo lettered by Professor His, Kin, and estimated as about 



twenty days old. 

 Fig. 195. — Embryo lettered by Professor His, Lr, and estimated as twenty or 



twenty-one days old. (Cf. Fig. 198, p. 491.) 



thick allantoic stalk, a.s. The head end of the embryo is well 

 developed, and raised freely above the yolk-sac ; but the body is 

 still so closely connected with the yolk-sac that a distinct yolk- 

 stalk can hardly be said to be present. The body of the embryo 

 is concave upwards, and the tail has a well-marked upward direc- 

 tion. In the neck, three visceral arches are visible as thicken- 

 ings, but the grooves between them are only faintly indicated- 



