THE DEVELOPMENT OF THE VASCULAR SYSTEM. 275 



replenishing the number. So far as the actual origin of new leucocytes in the 

 individual is concerned, our knowledge is very deficient. Some observers hold 

 that lymphocytes originate in the splenic pulp and in bone marrow; but from 

 what cells they originate is not certain. If the view stated in the preceding 

 paragraph expresses the real conditions, the other varieties of leucocytes are 

 direct descendants of the lymphocytes. It is undisputed that the lymphocytes 

 themselves proliferate in the follicles (nodules) of lymphatic organs. Mitotic 

 figures are almost invariably seen in the germinal centers of lymph follicles. 

 Centrosomes and mitotic figures have also been demonstrated repeatedly in the 

 other varieties of leucocytes, which fact seems to indicate that each variety of 

 leucocyte, when once established, is capable of reproducing itself. It has also 

 been suggested that the polymorphous nuclei indicate amitotic division. 



Before the appearance of bone marrow in the foetus, all varieties of leuco- 

 cytes may be found, but after the development of the marrow the formation of 

 the granular types becomes more active in this tissue. It seems possible that 

 the marrow is the only tissue in which the eosinophile, neutrophile and 

 basophile types are reproduced during adult life. 



If the leucocytes (leucoblasts) are derived primarily from mesodermal 

 (mesenchymal) cells, they have the same ultimate origin as the erythrocytes 

 (ery throblasts) . At first the mesodermal cells produce only erythroblasts; 

 later they produce leucoblasts also. These leucoblasts, or primitive wandering 

 cells, gain admission to the blood vessels by virtue o c their amoeboid activity. 

 It has been demonstrated that the lymphocytes (possibly identical with the 

 leucoblasts) possess, to a certain extent at least, the power of amoeboid move- 

 ment. And at this point it may be stated that the power of amoeboid movement 

 increases in the other varieties of leucocytes. 



One investigator believes that the ancestors of the leucocytes are formed 

 within the primitive blood vessels and stand in close genetic relation with the 

 vascular endothelium. 



Another investigator has thrown some doubt upon the mesodermal (mesen- 

 chymal) origin of the leucocytes by his studies on the thymus. He finds no 

 leucocytes in the blood before the appearance of the anlage of the thymus, but 

 finds them almost immediately after its appearance. He asserts that the 

 primitive leucocytes are apparently derived from cells which have invaginated 

 from the entoderm of certain of the branchial clefts to form the thymus. In 

 other words, the epithelial cells of the thymus are transformed into the primitive 

 leucocytes (Beard). 



The origin of the blood plates is even more obscure than the origin of the 

 blood cells, i. The theory that they represent products of disintegration of 

 leucocytes has not been corroborated. 2. The view that the plates stand in 

 genetic relation to the erythrocytes is supported by the fact that the latter can 



