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 protovertebre; - 
4 Forebrain 
2 Midbrain 
3 Hindbrain 
4 Auditory pit 
5 Viteline vein Oo Cerebral 
@ Future Hemisphere 
P nOvSTE GORA 1 Forebrain 
7 Medullary roldé 2 Midbrain 
8 Tart fold 5 3 Hindbrain ff 
9 Area pellucida 4 Auditory vesicle 
5 Heart 
6 Protovertebrae 
7 WNotochord 8 
8 Arteries A r 
9 False amnion ¥ 
lo Amnion 
4 Optic vesicle 
After Mills. 
Fig. 148.—Chick embryo thirty-six Fic. 149.—Chick embryo three 
hours old. 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 
