Irving Hardesty 247 



these mesodermal elements the capability of taking part in the develop- 

 ment of the neuroglia. In addition, Hatai, 02, observes in white rats 

 dividing cells of the endothelinm of the capillary walls and states that 

 some of the cells resulting from these divisions migrate into the sur- 

 rounding tissue. He thinks, further, that these migrating endothelial 

 cells become neuroglia cells. 



Thus it may be assumed from the above that there become distributed 

 in the central nervous system, in addition to the nuclei belonging to the 

 capillary wall proper, three forms of elements of mesodermal origin: 

 (1) Leucocytes or wandering cells, which usually but not necessarily 

 enter by way of the blood-vessels; (2) endothelial cells from the intima 

 of the capillaries; (3) nuclei belonging distinctively to the connective 

 tissue proper, which enters either as ingrowths of the developing pia 

 mater or secondarily from the externa of the blood-vessels. 



The assertion that these mesoblastic elements take part in the forma- 

 tion of the neuroglia is, I think, not wholly warranted but, at the same 

 time, it is difficult to refute it. The difficulty lies chiefly in the fact that 

 the mesodermal elements begin to enter at a time when no fibers are 

 differentiated and there is no way to characterize the connective-tissue 

 syncytium itself, and the consequent intermixing and fusing of the 

 syncytia from the two sources renders it well-nigh impossible, especially 

 in the outer layers of the specimen, to distinguish all the elements of 

 mesodermal origin from those which are not. In his study of the forma- 

 tion of the connective tissue of the body outside the central nervous 

 system, Maximow, 02, describes three forms of elements which are per- 

 haps identical with those mentioned above as contributed to the central 

 nervous system. He describes the leucocyte as the ordinary polymorpho- 

 nuclear variety and, in addition to the functions ascribed to it as such, 

 thinks it may change into other forms. The endothelial cells he speaks 

 of as " polyblasts " and, after discussing whether they can be considered 

 as really of the endothelium, he describes them, after their migration 

 from the capillaries, as similar to lymphocytes and as actively wandering 

 and phagocytotic. The third form is the " fibroblast," the pre-existing 

 connective-tissue corpuscle, directly concerned in the formation of white 

 fibrous connective-tissue. Maximow's observations are cited in order 

 to suggest the probability that the mesodermal elements in question may 

 play the same roles within the central nervous system as they do outside 

 it, that is, take part in the formation of connective tissue proper. How- 

 ever, the embryonic connective-tissue elements may be considered highly 

 responsive. To all appearances, very similar in the early embryos, they 

 later become so separately differentiated that some produce white or 



