II.J THE HEAD-FOLD. 27 



enclosed as formerly by the vitelline membrane alone, 

 comes to be also enclosed in a bag formed by the blasto- 

 derm. 



It is not however until quite a late period that the 

 complete closing in at the opposite pole takes place ; in 

 fact the extension of the blastoderm must be thought 

 of as going on during the first seven days 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. 



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. 



The embryo itself may be said to be formed by a 

 folding off 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 circum- 

 ference of the yolk, slightly but uniformly curved. Very 

 soon, however, there appears at a certain spot a semi- 

 lunar groove, at first small, but gradually increasing in 

 depth and extent; this groove, which is represented in 

 section in the diagram (Fig. 9, A), breaks the uni- 

 formity 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 itseif as a curved line (the hinder 

 of the two concentric curved lines in front of A in Fig. 

 22), which marks the hind margin of the groove, the 

 depression itself being hidden. 



