THE DEVELOPMENT OF YEKTEBRATA. 251 



of the blastoderm over the surface of the yolk. This is 

 effected by growth and division of the cells, which probably 

 now begin to absorb and assimilate some of the yolk. In a 

 day's time it has grown to three-quarters of an inch in 

 diameter ; in two days it covers half the surface of the yolk 

 and eventually (though much less rapidly) it extends over 

 the whole of the yolk. Meanwhile important changes are 

 taking place in it. 



We have already mentioned the division of the blastoderm 

 (as seen in surface view) into area pdlucida and area 

 opaca. It is the latter which more especially increases 

 in surface extent and surrounds the yolk; but it is from 

 the former only (and not from the whole of that) that the 

 embryo is developed. This statement may seem strange at 

 first, until the reader grasps the notion that the whole ovum 

 does not become the embryo in the case of the chick, but 

 that certain important structures (consisting of living tissues 

 continuous with those of the embryo) are formed outside 

 the embryo proper. These may be called extra-embryonic 

 structures, and are a feature not shown in the case of 

 Amphioxus and frog. 



The explanation of the existence of extra-embryonic 

 structures again lies in the enormous quantity of yolk. All 

 the frog's yolk can be kept in the cells of the mesenteron- 

 floor, and this enclosed by the body-wall. But with five 

 thousand times the bulk of yolk, enclosure within the body 

 of the embryo would involve impossible distortions of shape ; 

 hence most of the chick's yolk is outside its body in the 

 obvious anatomical sense. Morphologically, however, its 

 position is strictly the same as in the frog-embryo the 

 extra -embryonic structures which enclose it being really 

 portions of the mesenteron-wall and of the body- wall 

 extended beyond the limits of the future chick to avoid the 

 otherwise necessary distortions of form. These points will 

 become clearer as we proceed. 



6. Primitive Streak. As the blastoderm grows, the 

 area pellucida, at first circular, becomes pear-shaped, its 

 broad end marking the future anterior end. At the posterior 

 end, as already mentioned, there is a point where epiblast 



