86 PRELIMINARY ACCOUNT. [CHAP. 



remembered that the groove is at first crescent-shaped, 

 with the concavity of the crescent turned towards what 

 will be the hind end of the embryo (Fig. 22). 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 semicircular, 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 ?^ 

 (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 model, which he easily can 

 do by spreading a cloth out flat to represent the blasto- 

 derm, placing one hand underneath it, to mark the axis 

 of the embryo, and then tucking in the cloth from above 

 under the tips of his fingers. The fingers, covered with 

 the cloth and slightly projecting from the level of the 

 rest of the cloth, will represent the head, in front of 

 which will be the semicircular or horse-shoe-shaped 

 groove of the head-fold. 



At its first appearance the whole 8 may be sjDoken 



