BLOOD 



189 



The developing red blood corpuscles are known as erythroblasts, espe- 

 cially in their younger stages when the nuclei are reticular. In later 

 stages the nuclei become densely shrunken or pycnotic, and stain intensely 

 with haematoxylin. The entire cells become smaller, and are then called 

 normoblasts. The transition from an erythroblast to a normoblast is 

 shown in Fig. 180, a; during this process the cells divide repeatedly by 

 mitosis. 



It will be noticed that the terms applied to developing corpuscles are compounded 

 of words which describe the formative cells, instead of indicating what they produce. 

 Thus erythroblast signifies a red formative cell. Normoblast (Lat. norma, model or 

 type, and Gr. ^Xcwrtfy) is an objectionable term to designate a nucleated red corpuscle 

 of the usual size and form, in contrast with the large megaloblasts which occur in certain 

 diseases of the blood. Megaloblasts have reticular nuclei and presumably represent 

 a younger stage than the normoblasts. A reform in the nomenclature of blood cells 

 based upon morphological principles, is advocated by Minot (Human Embryology, 

 ed. by Keibel and Mall, 1912, vol. 2), and when agreement shall have been reached 

 regarding the relationships of the cells, it will be possible to adopt a reasonable 

 terminology. 



In becoming mature red corpuscles the normoblasts lose their nuclei. 

 Before they disappear, the pycnotic nuclei often assume mulberry, dumb- 

 bell, trefoil or other irregular shapes. 

 According to older observations they then 

 fragment, and are dissolved within the 

 normoblasts; but it is now generally be- 

 lieved that they are extruded from the 

 cells, either in one mass (Fig. 180, b) or 

 in detached portions, and that the ex- 

 truded nuclei are devoured by phagocytes. 

 The loss of the nuclei begins in human 

 embryos of the second month. In embryos 

 of the seventh month, nucleated corpuscles 



in the circulating blood have become infrequent, and after birth it is 

 rare to find one, except under pathological conditions. 



In withdrawing from the circulating blood the nucleated red corpuscles 

 do not disappear from the body. Since 1868 it has been known that the red 

 marrow, found within certain bones in the adult, contains an abundance 

 of erythroblasts, which multiply by mitosis. They are the source of the 

 new corpuscles constantly entering the circulation. In certain diseases 

 of the blood, imperfectly developed normoblasts also leave the marrow, 

 and circulate as in the embryo. Before the marrow assumes the blood- 

 forming function, the liver is the chief haematopoietic organ. Beginning 

 in embryos of about 7.5 mm., and continuing until birth, erythroblasts 

 are found between the hepatic cells and the endothelial cells of the sinu- 

 soids, and in certain stages they occur in vast numbers. Toward birth, 



FIG. 180. THE DEVELOPMENT OF RBD 



CORPUSCLES IN CAT EMBRYOS. 



(Howell.) 

 a, Successive stages in the development 



of a normoblast; b, the extrusion of 



the nucleus. 



