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 numljer of the protovertebrse ; 



I'orabratin 



IfindbreLin. 

 Atidit-oi^y pit 

 Viteline vw/n 



provofirehrAe 

 tfediclla-ry /bid 

 Tail /bid 

 Area. peZZitoida. 



Fig. 148.— Chick embryo thirty-six 

 hours old. 



Cerabral 



Hemi'Sphere 

 Forebr^a-in 

 Z^idbrain 

 Hindb-nain. 

 Auditory vaaicle 

 Heart 



Proto\^rtebr&e 

 J'/'otochorxi 

 Arteries 

 False s.7nn.ion. 

 Amnion 

 Optio yresiola 



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 



