274 



TEXT-BOOK OF EMBRYOLOGY. 



In the human embryo (he first non-nucleated disks or definitive erythrocytes 

 appear in the blood stream during the second month. From this time on, the 

 number gradually increases with a concomitant reduction in the number of 

 nucleated forms. At the time of birth, under normal conditions, the non- 

 rifUcleated disks constitute the only form of red blood cell found in the general 

 circulation. 



Since the erythrocytes are constantly dis'ntegrating and disappearing during 

 the life of the individual, there must be some means of replacing them. Ac- 

 cording to the theory of Maximow, a new supply is being constantly derived from 

 lymphocytes. Under normal conditions this process goes on for the most part, 



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; iv. /., wandering leucocytes (according 

 to Gulland, identical with first lymphocytes). 



probably wholly, in the bone marrow. Indifferent mesenchyme cells in the 

 marrow become differentiated into lymphocytes, some of which in turn give rise 

 to megaloblasts, normoblasts and erythrocytes in order, as described in the pre- 

 ceding paragraphs. In this way new erythrocytes are being constantly added 

 to the blood as the old ones perish, and an equilibrium is thus maintained. 



The question of the origin of the leucocytes is even more difficult than that 

 of the origin of the red blood cells. One view, of which Gulland has been the 

 strongest advocate, is that the lymphocytes, which are differentiated mesenchyme 

 cells (Fig. 251), represent the progenitors of the other varieties of white blood 

 cells, or in other words represent the youngest variety of leucocyte. According 

 to this view, the lymphocytes give rise to cells with a rather large amount of 



