2IO BLOOD AND BLOOD-FORMING ORGANS. 



M. Heidenhain (94) has made a particular study of the giant 

 cells. According to him the nuclei of these cells take the form of per- 

 forated hollow spheres whose thick walls contain " endoplasm." The 

 latter is continuous with the remaining protoplasm of the cell, the " exo- 

 plasm " through the "perforating canals" of the nuclear wall. The 

 exoplasm is arranged in three concentric layers, separated from each 

 other by membranes, the external membrane of the outer zone being the 

 membrane of the cell. The outer layer or marginal zone is of a transient 

 nature, but is always renewed by the cell. Thus, the cell-membrane is 

 replaced by the secondary membrane situated between the second and 

 third zone. According to the same author the functions of the giant 

 cells appear to consist in " the selection and elaboration of certain albu- 

 minoid substances of the lymph and blood currents, which are later 

 returned to the circulation." The number of centrosomes occurring in 

 the mononuclear giant cells of the bone-marrow is very large, and in 

 some cases, as in a pluripolar mitosis, may even exceed one hundred in 

 number. 



The distribution of the blood-vessels in the bone-marrow is as 

 follows : On entering the bone the nutrient arteries divide into a 

 large number of small branches, which then break, up into small 

 arterial capillaries. The latter pass over into relatively large venous 

 capillaries with relatively thin walls, which appear perforated in 

 certain places, so that the venous blood pours into the spaces 

 of the red bone-marrow where the current is very slow. The 

 blood passes out by means of smaller veins formed by the conflu- 

 ence of the capillaries which collect the blood from the marrow. It 

 is worth mentioning that the venous vessels, while inside of the 

 bone-marrow, possess no valves ; but, on the other hand, they 

 have an unusually large number of valves immediately after leaving 

 the bone. 



Yellow bone-mafrow is derived from red bone-marrow by a 

 change of the marrow-cells into fat-cells. The gelatinous marrow, 

 on the contrary, is characterized by the small quantity of fat which 

 it contains. Neither the yellow nor the gelatinous bone-marrow is 

 a blood-forming organ (compare Neumann, 90; Bizzozero, 91 ; 

 H. F. Miiller, 91 ; van der Stricht, 92). 



E. THE THYMUS GLAND. 



The thymus gland is usually considered as belonging to the 

 lymphoid organs, although in its earliest development it resembles 

 an epithelial, glandular structure. In the epithelial stage, this gland 

 develops from the entoderm of the second and third gill clefts. 

 Mesodermic cells grow into this epithelial structure, proliferate and 

 then differentiate into a tissue resembling adenoid tissue. It retains 

 this structure until about the end of the second year after birth, when 

 it slowly begins to retrograde into a mass of fibrous tissue, adipose 

 tissue, and cellular debris, which structure it presents in adult life. 



