308 



FORMATION OF THE EGG AND CHICKEN 



formed certain lateral plates, which later form the ribs. From 

 the above brief description it is evident that even during the first 

 day, many of the important structures of the body are clearly 

 outlined (Fig. 148). 



During the second day of incubation the remaining important 

 structures take shape, and those already formed develop further; 

 there is a decided increase in the number of the protovertebrse ; 



/ forebroiin 



2 ffidbrain, 



3 Ifindbrain, 



4 Auditory pit 



5 Vitelin& vein 

 G future 



7 Medullary 



8 Tail fold 



FIG. 148. Chick embryo thirty-six 

 hours old. 



FIG. 149. Chick embryo three 

 days old. 



the tubular heart and larger blood vessels are formed. Most 

 noticeable of all is the development in the region of the head of the 

 three cerebral vesicles at the extremity of the neural groove ; these 

 are called the anterior, middle, and posterior vesicle. In front of 

 them the developing cerebrum may be seen. The eye or optic 

 vesicle develops from the front cerebral vesicle, while the rudi- 

 mentary ear and the auditory pit are formed from the posterior 

 vesicle. The early formation of the heart and the rapid develop- 

 ment of the vascular system are important features of the second 



