THE POURTH WEEK. 



493 



tinuouB structures, but the enlargements of the ridges, which 

 become later the arms and legs, are rapidly increasing in size. 

 Towards the end of the fourth week (Figs. 202, 203, 204), 



Fig. 199. 



Fig. 200. 



Fig. 201. 



Fig. 199-201. — Outline figores of three Human Embryos, estimated as about 

 twenty-three days old. (From His.) x 5. 



Fig. 199. — Embryo figured and described by Coste.- For a figure of the uterus 



and blastodermic vesicle from which this embryo was obtained, see Fig. 



255, p. 608. 

 Fig. 200. — Embryo lettered by Professor His a. 

 Fig. 201. — Embryo figured and described by AUen Thomson. By a mistake 



of the engraver's this figure has been reversed ; and it is really the right 



side of the embryo, and not the left, that is shown. 



the embryo acquires a very characteristic form, corresponding 

 closely in shape, in size, and in internal structure with a chick 



Fig. 202. 



Fig. 203. 



Figs. 202 and 203.— Outline figures of twc Human Embryos, estimated as 

 twenty-seven days old. (From His.) x 5. 



Fig. 202.— Embryo lettered by Professor His, B. 

 Fig. 203. — Embryo lettered by Professor His, A. 



embryo at the end of the fourth day, or a rabbit embryo of the 

 eleventh day. 



The embryo (Fig. 204), which measures 7-5 mm. along its 

 longest diameter, is still strongly flexed. The back is rather 

 straighter than before, but, owing to the very sharp bend in 



