HISTOGENESIS OF CIRCULATORY SYSTEM 735 



terrestrial form, Rana temporaria, the bone marrow functions in this capacity 

 as it does in the adults of reptiles, birds, and mammals. In the adult reptile and 

 bird, the bone marrow seems to function in the production of all types of blood 

 cells. In the mammal, the bone marrow possibly elaborates only erythrocytes 

 and granular, white blood cells, while the lymphocytes probably are produced 

 in other areas, such as the pharyngeal and palatine tonsils and lymph nodes, 

 etc. In all vertebrates from the teleost fishes to the mammals, it is probable 

 that scattered lymphoid tissue in various parts of the body functions in the 

 formation of lymphocytes. 



During the development of the early human embryo and later fetus, the 

 following have been given as sites of blood-cell formation (Minot, '12; 

 Gilmour, '41 ) : 



(a) yolk sac in embryos up to 3 mm., i.e., the end of the fourth week 

 of pregnancy, 



(b) mitosis of previously formed erythroblasts in general circulation, yolk 

 sac, and chorion of embryos from 3 to 9 mm. in length, 



(c) liver and yolk sac of 10- to 18-mm. embryos. In embryos of 470- to 

 546-mm. there is a gradual decrease in the liver, 



(d) spleen, beginning in the 28-mm. embryo; thymus, and lymph glands 

 in the 35-mm. and larger embryos, 



(e) bone marrow during the third month and later. 



3) Characteristics of Development of the Erythrocyte. Most vertebrates 

 in the adult condition retain the nucleus in the erythrocyte or red blood cell. 

 To this cell is given the function of carrying oxygen from the site of external 

 respiration to the body tissues. It also is a factor in conveying carbon dioxide 

 from the tissues to the site of external respiration. The oxygen-carrying capacity 

 of the erythrocyte resides in the presence of the compound hemoglobin. 

 Hemoglobin is a complex protein molecule, containing iron atoms. The iron 

 atoms make it possible for the hemoglobin to convey oxygen. 



In the adults of various amphibian species, there is a tendency for the 

 red blood cell to lose its nucleus by various means (Noble, '31, pp. 181-182). 

 This tendency toward loss of the nucleus reaches an extreme form in Batra- 

 choseps where more than 90 per cent of the red blood cells have lost their 

 nuclei. In adult mammals, the mature erythrocyte loses its nucleus (column 

 6, fig. 333 A) but it is retained in the early embryo. 



4) Characteristics of Various White Blood Cells. White blood corpuscles 

 or leukocytes vary greatly in number and in morphological features in all 

 vertebrates. In general, the following two major groups of white blood cor- 

 puscles may be distinguished. 



a) Granulocytes. Granulocytes are cells which arise from granuloblasts 

 (columns 3, 4, and 5, fig. 333A). These cells are characterized by the 



