274 



TEXT-BOOK OF EMBRYOLOGY. 



For a time after their first appearance, the erythroblasts are the only blood 

 cells. Then other cells appear in the blood, which are the forerunners of the 

 leucocytes and are known as leucoblasts. The origin of the leucoblasts is not 

 known. They probably arise outside of the blood vessels and enter the blood 

 stream secondarily. It has been suggested that they are differentiated from 

 mesodermal (mesenchymal) cells in connection with the origin of the lymphatic 

 vessels (p. 280; also Fig. 251). In this case they would not improbably be 

 identical with the first lymphocytes. But whether the lymphocytes in the blood 

 and in the lymph represent the progenitors of the other varieties of leucocytes 

 has not been determined. There is, however, a very plausible view that the 



FIG. 251. From section of groin of human embryo of about 7 weeks. Gulland. 

 c. t. n., Connective (mesenchymal) tissue nuclei; ly., lymphatic vessel; tr., trabecula or strand of 

 mesenchymal tissue between two lymphatic vessels; w. /., wandering leucocytes (according 

 to Gulland, identical with first lymphocytes). 



lymphocytes do represent the youngest variety of leucocyte; that they give rise 

 to cells which possess a rather large amount of finely granular cytoplasm 

 mononuclear leucocytes; that the nuclei of the mononuclear forms become drawn 

 out into horse-shoe shapes characteristic of the transitional variety; that the 

 nuclei of the transitional forms then become lobed or separated into two or more 

 parts to form the nuclei of the polymorphonuclear or polynuclear varieties. Ac- 

 cording to this view, therefore, the different varieties of leucocytes bear definite 

 genetic relations to one another, and the polymorphonuclear and polynuclear 

 varieties indicate senility and the beginning of degeneration. 



In the case of the leucocytes, as in the case of the erythrocytes, the oldesi 

 cells are constantly disappearing during life, and there must be some means of 



