XXXIX. 



DEVELOPMENT OF THE FIFTH DAY 



On this day the head and tail of the embryo have nearly come to- 

 gether by the great curving of the chick. The yolk is completely cov- 

 ered by the blastoderm, and the vascular area covers nearly two-thirds 

 of the blastoderm. 



THE LIMBS 



It is during this day that the limb-buds increase considerably in size, 

 and are marked off into a proximal rounded portion and an expanded 

 distal region. It is in the expanded distal region that the digits can be 

 seen to form in cartilage. The rounded proximal portion is slightly bent 

 at the points where elbow and knee joints will be formed. 



The elbow and knee-angles at first are directed almost straight out 

 from the body, but on about the eighth day both fore and hind-limbs 

 rotate until the elbow-joint points caudad, while the knee-joint points 

 cephalad. 



By the end of the tenth day, both pairs of appendages have their 

 definite outlines, though feathers and nails are not yet formed. 



Although the structures which are to become bones are first out- 

 lined in cartilage, they later become ossified. There are three well- 

 formed digits in the expanded distal portion of the fore-limb at this time 

 with a possible fourth in a rudimentary condition, while in the expanded 

 distal portion of the hind-limb there are also three well-defined digits 

 with two in a rudimentary condition. 



The development of the bony vertebrae has already been discussed. 

 Here it is well to state that the ribs develop as cartilaginous bars in the 

 body wall of the chick. The ventral ends of these fuse ventrally, and 

 after fusion, a portion of each of the fused ends separates from the re- 

 maining ribs from which they formed. It is this portion which has sep- 

 arated that becomes the sternum. 



THE DEVELOPMENT OF THE SKULL 



The skull is divided into two regions: (1) The skull proper, and 

 (2) the visceral skull, this latter being that portion which has developed 

 from the visceral arches. 



THE SKULL PROPER 



The notochord forms a sort of central portion around which the 

 vertebrae form. The anterior end of the notochord serves a sort of 

 similar function in the head region. 



