Development of the Second Day 149 



and the cells soon begin to multiply rapidly by 

 mitotic division. 



The origin of the first white corpuscles is 

 still uncertain. In the chick they do not ap- 

 pear until about the eighth day ; and it seems 

 probable that they have no real relationship to 

 the red corpuscles. It is probable that they 

 are of several kinds and have several distinct 

 origins. 



By the development of buds from the ves- 

 sels already formed the vascular area continues 

 to increase in extent. Some of these buds, 

 during the second day, begin to grow towards 

 and into the embryo, through whose tissue they 

 work their way along certain prescribed paths. 

 While this growth is going on, certain of the 

 vessels increase in size and become arteries or 

 veins, while other vessels remain small as capil- 

 laries. Certain of these larger vessels unite 

 with the posterior end of the heart, which has 

 already been formed and has begun to beat ; 

 and other vessels unite with the anterior end 

 to form the larger vessels of the arterial system. 

 Since the heart has begun to beat, and the 

 corpuscles have already been formed, the cir- 

 culation is established as soon as the blood 

 vessels become connected with the heart. 



