EVOLUTION OF THE CHICK. 151 



marrow itself, becoming* enlarged, forms the brain. 

 At this period the fluid surrounding the foetus be- 

 comes more consistent arid less transparent. About 

 the same time also the spine, which was at first ex- 

 tended in a straight line, becomes bent, and the joints 

 of the bone (vertebrae) become distinct. The eyes 

 are distinguished by their black pigment and com- 

 paratively large size, as they afterwards are in conse- 

 quence of a peculiar slit in the lower part of the iris, 

 a circumstance also observable in the nimble lizard 

 (Lacerta agilis) and other animals which have no 

 pupillary membrane. 



On the fourth day the pupil of the eye can be dis- 

 tinguished as well as the aqueous and vitreous hu- 

 mours. In the head are perceived five vesicles, filled 

 with a fluid; and these as they enlarge approach each 

 other, coalesce, and form the brain, invested with its 

 membranes. The wings also grow, the thighs begin 



An Egg opened four days after incubation, with a magnified view of the 

 Chick. 



