DEVELOPMENT OF THE CHICK 143 



blood and the richest in nutriment is that 

 which is carried to the heart by the caudal 

 vena cava. 



Conformation of the Embryo. — The external 

 form of the embryo may be said to be dominated 

 by three sets of factors. (1) In the early 

 stages of development the central nervous 

 system and the somites produce the elongated 

 and curved character of the early embryo. 

 (2) With the appearance of viscera, such as 

 the heart, intestines, and liver, the external 

 embryonic features are altered. (3) The de- 

 velopment of the skeleton and muscles serves 

 to produce definite avian external character, 

 which gradually mask the outlines of the 

 internal organs. 



Prior to the sixth day the embryo presents 

 no features by which it can be distinguished as 

 avian in character. On the third or fourth day 

 (Fig. 71) the whole embryo is much bent upon 

 itself. A cervical flexure is very pronounced, 

 but there is no definite neck. The brain gives 

 distinct prominence to the head, especially 

 in the region of the mid - brain. Eyes are 

 present, and there are visceral clefts. The 

 heart produces an external protuberance, and 

 limbs are represented by bud-like outgrowths. 



