DEVELOPMENT OP THE CHICK SECOKD PEEIOD. 317 



up on the fifth day ; at the same time the first is lost en- 

 tirely ; but the second 

 continues longer open 

 (fig. 342, A, y). On the 

 third and fourth days, 

 the part of the ventral 

 lamina, which is situated 

 in front of the lower jaw 

 thickens and resolves 

 itself into the upper 

 jaw (fig. 341, r, and 

 345, 1 above 2); this 

 part is more strongly 

 marked on the fifth day 

 (fig. 342, A, c). The 

 two sides of the upper 

 jaw do not meet in the 

 first instance ; they co- 

 alesce at a later period, 

 through the medium of 

 thefrontalprocess, which 

 is developed betwixt the 

 eyes (fig. 342, B, over 

 D). 



The rudiments of the 

 ribs begin to be formed 

 in the parts of the ven- 

 tral laminae lying behind 

 the branchial arches ; 

 the extremities show 



Fig. 345. Embryo of the fowl of the 

 first half of the fourth day ; o, corpora 

 quadrigemina ; 6, hemispheres ; c, meso- 

 cephalon (thalami) ; d, fourth ventricle ; 

 /, eye, the cleft in the choroid beginning 

 to close ; p 1 , g 1 *, the first and second 

 branchial spaces still entirely open ; g 3 , ^*, 

 the third and fourth spaces open be- 

 hind only ; h, the ventricle of the heart, 

 now of a rounded form ; i, aorta ; n, al- 

 lantois ; o, anterior, and p, posterior ex- 

 tremity. 1, 2, Upper and under jaw. 

 The line with the asterisk indicates the 

 natural length of the embryo. 



themselves upon the ex- 

 ternal aspects of the same laminae. Of the extremities there is 

 still no trace to be discovered in the first half of the third day 

 (fig. 339), but in the second half of that day they arise on 

 the sides of the ventral laminae as narrow edgings, which by 

 the close of the day have turned more upwards, gained the 

 outer margins of the ventral laminae, and changed into 

 rounded offsets (fig. 341, o, p}, the posterior pair being dis- 

 tinguished from the anterior by somewhat greater breadth 

 (fig. 345, o, p) ; on the fifth day they recede still more up- 



