XIV 



THE LYMPH 



535 



those that issue from the bone are furnished with an extraordinary 

 number of them. 



The cells contained in the lacunar system of the marrow are 

 quite characteristic of this tissue. Among them four principal 

 kinds can be distinguished : leucocytes, megacaryocyfces (giant 

 cells), erythroblasts, erythrocytes. 



The leucocytes of bone marrow comprise many varieties, which 



FIG. 252. Leucocytes from dog's bone marrow, dry preparation, a, //, Young leucocytesior 

 lymphocytes, with giant nuclei and little cytoplasm ; c, d, medullary cells or adult leucocytes 

 with reniform and polymorphous nuclei ; e, leucocytes undergoing mitotic division. 



probably represent different states of development of a single 

 cellular type, since there are always numerous transitional forms 

 from the one to the other variety. The differences lie in the 

 dimensions, form of nucleus, 

 and character of cytoplasm. 

 The youngest (lymphocytes) 

 are the smallest, owing par- 

 ticularly to the paucity of pro- 

 toplasm around the nucleus ; 

 the disc -shaped nucleus is 

 rich in chromatin. The more 

 adult (medullary cells) are 

 larger, with a reniform 

 polymorphous, sometimes 

 multiple nucleus, poor in 

 chromatin. These are not 

 found in normal circulating 

 blood, only in states of leu- 

 caemia (Fig. 252). 



Howell's megacaryocytes 

 were discovered in 1869 by 

 Bizzozero, who called them giant cells with a 

 nucleus. They have an average diameter of 25-45 //. Their 

 nucleus is very variable in form, often horseshoe-shaped. Heiden- 

 hain distinguishes several types or varieties according to the different 

 degree of differentiation of the cytoplasm, which is sometimes 

 arranged in three concentric zones (Fig. 253). Bizzozero had 

 already suggested that the giant cells were derived from the 

 leucocytes of bone marrow, since there is a whole series of forms 



FIG. 2;13. Megacaryocyte of bone marrow, in which 

 a large horseshoe nucleus can be distinguished 

 from the cytoplasm, divided into three concentric 

 zones. (Heidenhain.) 



budding central 



