34 PRELIMINARY ACCOUNT. [CHAP. 



their bays, instead of being shallow cups, would become more 

 tubular. Such a result is in part arrived at by the growth 

 of the blastoderm; the upper limb of the 8 is continually 

 growing forward (but, unlike the stretched elastic model, in- 

 creases in all its dimensions at the -same time), and the lower 

 limb is as continually lengthening backwards ; and thus both 

 upper and lower bays become longer and longer. This we 

 shall hereafter speak of as the travelling backwards of the 

 head-fold. 



The two bays do not however both become tubular. The 

 section we have been speaking of is supposed to be taken 

 vertically along a line, which will afterwards become the axis 

 of the embryo ; and the lower bay of the 3 is a section of the 

 crescentic groove mentioned above, in its middle or deepest 

 part. On either side of the middle line the groove gradually 

 becomes shallower. Hence in sections taken on either side 

 of the middle line or axis of the embryo (above or below the 

 plane of the figures), the groove would appear the less marked 

 the farther the section from the middle line, and at a certain 

 distance would disappear altogether. It must be remembered 

 that the groove is at first crescent-shaped, with its concavity 

 turned towards what will be the hind end of the embryo 

 (Fig. 11). As the whole head-fold is carried farther and farther 

 back, the horns of the crescent are more and more drawn 

 in towards the middle line, the groove becoming first semi^ 

 circular, then horse-shoe-shaped. In other words, the head- 

 fold, instead of being a simple fold running straight back- 

 wards, becomes a curved fold with a central portion in front 

 running backwards, and two side portions running in towards 

 the middle line. The effect of this is that the upper bay of 

 the 8 (that within the embryo) gets closed in at the sides 

 as well as in the front, and thus speedily becomes tubular. 

 The under bay of the 8 (that outside the embryo) remains 

 of course open at the sides as in front, and forms a sort of 

 horse-shoe-shaped ditch surrounding the front end of the 

 embryo. 



We have dwelt thus at length on the formation of the 

 head-fold, because, unless its characters are fairly grasped, 

 much difficulty may be found in understanding many events 

 in the history of the chick. The reader will perhaps find the 

 matter easier to comprehend if he makes for himself a rough 



