28 PRELIMINARY ACCOUNT. [CHAP. 



completely envelopes it. Thus the whole yolk, instead of 

 being enclosed as formerly by the vitelline membrane alone, 

 comes to be also enclosed in a bag formed by the blastoderm. 



It is not however until quite a late period that the 

 complete closing in at the opposite pole takes place, so 

 that the extension of the blastoderm must be thought of as 

 going on during nearly the whole period of incubation. 



Both the area opaca and the area pellucida share in 

 this enlargement, but the area opaca increases much more 

 rapidly than the area pellucida, and plays the principal 

 part in encompassing the yolk. 



4. The mesoblast, in that part of the area opaca which 

 is nearest to the area pellucida, becomes the seat of peculiar 

 changes, which result in the formation of blood-vessels. 

 Hence this part of the area opaca is called the vascular area. 



5. The embryo itself may be said to be formed by a 

 folding off of the central portion of the area pellucida from 

 the rest of the blastoderm. At first the area pellucida is 

 quite flat, or, inasmuch as it forms part of the circumference 

 of the yolk, slightly but uniformly curved. Very soon, how- 

 ever, there appears at a certain spot a semilunar groove, at 

 first small, but gradually increasing in depth and extent ; this 

 groove, which is represented in section in the diagram (Fig. 

 8, A) 9 breaks the uniformity of the level of the area pellucida. 

 It may be spoken of as a tucking in of a small portion of the 

 blastoderm in the form of a crescent. When viewed from 

 above, it presents itself as a curved line (the hinder of the 

 two concentric curved lines in front of A in Fig. 11), which 

 marks the hind margin of the groove, the depression itself 

 being hidden. In a vertical longitudinal section carried 

 through the middle line, we may recognize the following 

 parts (Fig. 8, A, or on a larger scale Fig. 9, which also 

 shews details which need not be considered now). Beginning 

 at what will become the posterior extremity of the embryo, 

 (the left-hand side of the figure in each case), and following 

 the surface of the blastoderm forwards (to the right in the 

 figures), the level is maintained for some distance, and then 

 there is a sudden descent, the blastoderm bending round and 

 pursuing a precisely opposite direction to its previous one, 

 running backwards instead of forwards, for some distance. 

 It soon however turns round again, and once more running 



