480 EMBRYOLOGY OF THE CHICK 



At about thirty-eight hours, the three primary vesicles divide to 

 form five vesicles (Fig. 278, 282). 



The prosencephalon divides into telencephalon (end-brain), and 

 diencephalon ('twixt-brain) ; the mesencephalon remains undivided, while 

 the rhombencephalon divides into metencephalon (cerebellum and 

 pons), and the myelencephalon (medulla oblongata). 



The telencephalon has not yet completely separated from the dien- 

 cephalon, but there is a median enlargement showing where the division 

 will take place. 



The two most anterior neuromeres of the original rhomencephalon 

 form the metencephalon, and the posterior four neuromeres form the 

 myelencephalon. 



At thirty-five hours, the auditory pits begin growing as thickened 

 ectoderm known as auditory placodes on the dorso-lateral surface oppo- 

 site the most posterior inter-neuromeric construction of the myelen- 

 cephalon. At thirty-eight hours, the general level of the ectoderm has 

 become depressed to form a pair of cavities known as auditory pits. The 

 pits seem to recede until they become closed vesicles, and separate from 

 the superficial ectoderm, although it will not be until later that they 

 form a definite connection with the central nervous system. 



TORSION 



At about the same time the cephalic flexure begins, there is also the 

 beginning of a twisting of the entire embryo (Fig. 280), although at 

 this time the twisting is only observable in the head region. The bend- 

 ing of the cephalic region downward is, as already stated, called 

 "flexion." The twisting of the embryo from its ventral aspect to its side 

 is known as "torsion." 



As the yolk lies directly beneath the embryo, it can easily be under- 

 stood that any bending ventrad would be stopped by the large mass of 

 inert yolk beneath, so that if there is to be any considerable bending at 

 all, the entire embryp must turn on its side, and this it does in all eggs 

 which possess considerable yolk, though it does not necessarily come to 

 lie on the same side in all amniotes. The chick turns so that its left side 

 lies next to the yolk. 



Torsion begins in the head region and gradually and slowly extends 

 the full length of the body, so that a whole mount, after torsion is com- 

 pleted, shows the embryo lying on its left side with head and tail close 

 together, the entire embryo forming from one-half to about three-fourths 

 of a circle. 



THE CIRCULATORY SYSTEM 



By the end of the second day, the heart has become still more 

 twisted, and is now S-shaped with the venous end above and behind the 

 arterial end, so that both ends lie close together with the loop as an 



