158 



COMPARATIVE PHYSIOLOGY. 



The History of the Blood-Cells. 



"We have already seen that the Ijlood and the vessels in 

 ■which it flows have a common origin in the mesoblastic cells of 

 the embryo chick ; the same applies to mammals and lower 

 groups. The main facts may be grouped under two head- 

 ings : 1. Development of the blood-corpuscles during embry- 

 onic life. 2. Development of the corpuscles in post-embryonic 

 life. The origin and fate of the corpuscles, especially of the 

 colored variety, have been the subject of much discussion. 



The best established 

 facts are stated in the 

 summary below, while 

 they are illustrated by 

 the accompanying fig- 

 ures. 



The colorless cells 

 of the blood first arise 

 as migrated undifferen- 

 tiated remnants of the 

 early embryonic cell 

 colonies. That they re- 

 main such is seen by 

 their physiological be- 

 havior, to be considered 

 a little later. Afterward 

 they are chiefly pro- 

 duced from a peculiar 

 form of connective tis- 

 sue known as leucocy- 

 tenic, and which is 

 gathered into organs 

 (lymphatic glands), the 

 chief function of which is to produce these cells, though this 

 tissue is rather widely distributed in the mammalian body in 

 other forms than these. 



Summary. — ^The student may, with considerable certainty, 

 consider the colorless corpuscle of the blood as the most primi- 

 tive; the red, derived either from the white or some form of 

 more specialized cell ; the nucleated, as the earlier and more 

 youthful form of the colored corpuscle, which may in some 

 groups of vertebrates be replaced by a more specialized (or de- 



FiQ. 143.— Surface view from below of a email por- 

 tion of posterior end of pellucid area of a chick 

 of tliirty-six hours, 1 x 400 (Foster and Balfour). 

 J. c, blood-corpuscles ; a, nuclei, which subse- 

 quently become nuclei of cells forming walls of 

 blood-vessels ; j). pr, protoplasmic processes, 

 containing nuclei with large nucleoli, n. 



