chap, ii.] BLOOD. 15 



cleavage of the white corpuscles of the blood of lower 

 vertebrates has been directly observed by Klein and 

 Ranvier. 



15. In every microscopic specimen of the blood 

 of man and mammals are found a variable number 

 of large granules, more or less angular, singly or in 

 groups, which have been specially studied by Osier. 

 According to Bizzozero they are, when 



observed in the living and fresh blood, O 

 pale, circular, or slightly oval discs 

 (Fig. 12, 6). Their size is only \ to \ 

 of that of the red blood corpuscles. 

 They are called by him blood -plates, and 

 he supposes them to be of essential im- Flg ' 

 portance in the coagulation of the blood, 

 originating the fibrin ferment. Hayem 

 described them previously as being in- 

 termediate forms in the development of red blood 

 corpuscles, and called them hsematoplasts. 



16. Development of Blood Corpuscles. 

 At an early stage of embryonic life, when blood 

 makes its appearance it is a colourless fluid, contain- 

 ing only white corpuscles (each with a nucleus), which 

 are derived from certain cells of the mesoblast. These 

 white corpuscles change into red ones, which become 

 flattened, and their protoplasm gets homogeneous and 

 of a yellowish colour. All through embryonic life new 

 white corpuscles are transformed into red ones. In 

 the embryo of man and mammals these red corpuscles 

 retain their nuclei for some time, but ultimately lose 

 them. New nucleated red blood corpuscles are, how : 

 ever, formed by division of old red corpuscles. Such 

 division has been observed even in the adult blood of 

 certain lower vertebrates (Peremeschko) as well as in 

 the red marrow of mammals (Bizzozero and Torre). 



An important source for the new formation of red 

 corpuscles in the embryo and adult is the red marrow 



