2 HISTOLOGY 



of our universe are invariably mostly concerned, carbon, oxygen, 

 hydrogen, and nitrogen; and that secondly, smaller quantities of sul- 

 phur, phosphorus, iron, etc., are found, some of them only occasionally, 

 in its mass. The four main elements occur in apparently constant quan- 

 tities: carbon 45 parts; oxygen 28 parts; hydrogen 8 parts; nitrogen 

 15 parts; and phosphorus 4 parts, which leads one to suppose them united 



FIG. i. Ovarian ovum of a cat just before maturity, c.m., cell membrane; n.m, nuclear 

 membrane; nd., nucleolus; mics., microsomes; yk.al., yolk alveoli. 



in a chemical formation in which the second group of elements apparently 

 do not take direct part except, perhaps, the phosphorus. 



Turning for light to the structure, it is seen that the protoplasm 

 in a given protoplast or cell is not a homogeneous mass; it is differen- 

 tiated into several organs of a considerable size, as a nucleus, centrosome, 

 plastid, nucleolus, etc. (Fig. i), while in finer structure it is evidently 

 composed of several, at least two or three, substances arranged in some 

 sort of a tissue of threads, network, or alveolus, mixed with granules. 

 This finer structure has been described by many investigators as either 



