PROGRESS FROM THE GENERAL TO THE SPECIAL. 



927 



327) an upper, middle, and inferior termed respectively the epiblast (A), 

 mesoblast (B), aud hypoblast (c). It also begins to spread circularly over the 

 vitellus, so that instead of appearing as a mere disk it forms an investment 



FIG. 328, 



Hl.6 



Section of a Blastoderm at right angles to the long axis of the Embryo after eight hours' incubation. 

 Taken about midway between the front and hind end. A, Epiblast; B, Mesoblast; c, Hypoblast; pr, 

 primitive groove ; /, fo!d in the blastoderm, probably produced by the action of the chromic acid ; me, 

 mesoblast cell the line points to one of the peripheral mesoblast ceils lying between epiblast and hy- 

 poblast: bd, formative cells. The following are the chief points represented in the section: (1) The 

 thickening of the mesoblast underneath the primitive groove pr, even when it is hardly at all present 

 at the sides of the groove ; (2) The hypoblast, c, early formed as a single layer of spindle-shaped cells; 

 <3) The so-called segmentation cavity, in which coagulated albumen is present. On the floor of this 

 are the large formative cells & d. The line of separation between the epiblast and mesoblast is too 

 strongly marked in the figure. 



for the yolk beneath the vitelline membrane, which, however, only becomes 

 complete towards the close of the period of incubation. The bag thus pro- 

 duced is chiefly formed by the growth 

 of the area opaca, but in part, also, by 

 that of the area pellucida, which gradu- 

 ally assumes an oval and then a pear- 

 shape. In the centre of the area pellu- 

 cida a longitudinal thickening is now 

 seen, termed the primitive streak, which 

 5s formed by the multiplication of the 

 cells both of the epiblast and of the 

 mesoblast, but chiefly of the mesoblast, 

 and this, again, is quickly marked by a 

 dimple, the primitive groove (pr, Fig. 

 328). This is the first indication of the 

 embryo, which in its further develop- 

 ment is formed by a folding off' of the 

 area pellucida from the rest of the blas- 

 toderm; though it may be mentioned 

 incidentally that the primitive groove 

 early disappears without entering di- 

 rectly into the formation of any part of 

 the embryo. The first indication of the 

 folding off of the embryo consists in the 

 formation of a crescentic furrow (Fig. 

 328, the first dark crescent above the 

 line A) at a point which corresponds to 

 the head of the embryo, and very shortly 

 afterwards tail aud lateral folds form, 

 which give rise more and more distinctly 

 to the appearance of a small tubular sac seated upon and connected by a 



Surface view of the Pellucid Area of a Blas- 

 toderm of 18 hours. None of the opaque area 

 is shown, the pear-shaped outline indicating 

 the limits of the pellucid area. . 



