126 WALTER HE APE. 



optic vesicles ; these are shown as two bud-like vesicles pro- 

 jecting outwards and backwards, and slightly downwards from 

 the front end of the neural tube ; behind them the swelling of 

 the fore-brain is discernible, while still further backwards and 

 at the edge of the body of the embryo the two tubes of the 

 heart are indicated. 



The folding off of the embryo from the yolk-sac has at this 

 stage made some progress, and, indeed, the whole of the head 

 of the embryo as far back as the line so. pi. now lies projected 

 freely above the blastodermic vesicle. 



Stages H and J.^ — These stages are depicted in figs. 7 and 9, 

 the embryo represented in the former figure being 2*2 mm. 

 long, that in the latter figure 3"06 mm. long. The more com- 

 plete closure of the medullary canal and the constriction of its 

 anterior region into fore-, mid-, and hind-brains is to be noticed. 

 The optic vesicles are still seen in fig. 9; in fig. 7 they are 

 barely noticeable, owing to the curved position of the embryo 

 when drawn. 



The increase of the protovertebrse and the gradual reduction 

 of the sinus rhomboidalis is also seen, while the thickened 

 anterior end of the primitive streak is now enclosed within 

 the posterior walls of the medullary canal, and projects up- 

 wards as a rounded knob at its hinder end. 



The direction of the increase of the protovertebrae is a 

 difficult matter to determine, but a careful examination and 

 measurement of figs. 5, 7, and 9 leads me to believe that in all 

 probability the increase is almost altogether posteriorwards 

 during those stages. The embryo (fig. 7) of Stage h has, 

 however, apparently one protovertebrae more anteriorly than 

 the embryo of Stage r (fig. 5), and the embryo (fig. 9) of 

 Stage J one more than that of Stage h (fig. 7). The embryos 

 of Stages E and f are more difficult to compare (figs. 1 and 

 5), but I think it is highly probable the increased number in 

 the latter is due to a backward growth. 



The amnion at Stage h completely covers the embryo (fig. 

 7), an anterior limb having grown over the head as the 



