284 OUTLINES OF CHORDATE DEVELOPMENT 



folded off from the yolk and blastoderm. The embryo of 

 thirty hours was nearly straight and lay flat upon the yolk, 

 save that a simple flexure of the head was caused by the growth 

 of the fore-brain downward in front of the fore-gut. Continued 

 enlargement of the head (brain) leads to two important general 

 modifications of the form of the embryo, first, a twisting of the 

 embryonic body, and second, the formation of certain bendings 

 or flexures in the axis of the head and trunk. As the cavities 

 of the brain enlarge, a sharp bending of the neural axis appears 

 between the fore- and mid-brain, soon involving the entire 

 mid-brain. The development of this cranial flexure (Fig. 112), 

 combined with the extensive enlargement of the fore-brain, 

 would force this region down into the yolk but for the fact that 

 the head now falls over upon one side, its left, which now comes 

 to lie over upon the yolk. The trunk retains for a time its 

 original position flat upon the yolk, and thus a twist appears 

 just back of the head. This change in the position of the head 

 begins about the thirty-sixth hour (15 pairs of somites), the 

 cranial flexure having appeared somewhat earlier, about the 

 thirtieth hour (10-12 pairs of somites). Gradually the region 

 of the twist moves posteriorly, leaving the more anterior parts 

 thrown over (Figs. 113, 114), and finally, about the close of 

 the fourth day, the entire embryo comes to lie upon its left side. 

 Long before this, about the end of the second day, another 

 flexure of the longitudinal axis appears; this is the cervical 

 flexure. A broad ventral sweep through the entire hind-brain 

 region, extending a short distance into the neck, results in the 

 more complete flexure of the fore-brain, which thus comes to 

 be directed posteriorly rather than ventrally, its true ventral 

 surface approximating the ventral surface of the pharyngeal 

 region. Later the entire trunk becomes arched dorsally, 

 bringing the head and tail fairly close together on the ventral 

 side (the morphologically ventral side now lies turned toward 

 what was the right side, previous to the torsion of the embryo). 

 During the later phases of embryonic development these last 

 flexures disappear to a considerable extent, leaving permanently 

 well-marked only the original cranial flexure. 



