1901.] MONTGOMERY — CELLULAR BASIS OF HEREDITY. 7 



ing the thinly fluid nuclear sap, which fills it, is a delicate network 

 or^meshwork of linin threads, and supported upon or imbedded in 

 them masses of a substance called chromatin. In the nuclear sap 

 may be suspended also one or many large rounded bodies, the 

 nucleoli, and numerous minute lanthanin granules. The whole is 

 enclosed by a nuclear membrane. 



The paternal germ cell, the spermatozoon, has a very different 

 appearance, and in volume is exceedingly smaller than the ovum. 

 In the case of the sea-urchin, Wilson {The Cell in Development and 

 Inheritance^ has computed it to be about one half-millionth the 

 volume of the Qgg, and the difference is many times greater than 

 this in the case of the bird. The history of its formation shows it 

 to be a highly specialized cell with regard to its cytoplasm, which 

 is generally modified to form a locomotory flagellum. But its 

 amount of chromatin is the same as that in the ^gg cell, though 

 contained in a very condensed form (composing the head of the 

 spermatozoon). At the junction of the flagellum and head there 

 is frequently found a mid-body, a metamorphosed centrosome. 

 Thus there is a division of labor between the two germ cells : the 

 ovum is large to provide the necessary cytoplasm and nourishment 

 for the embryo ; the spermatozoon minute and motile in order to 

 reach the ovum. 



All cell reproduction is by division of the cell, and the mode of 

 division, which differs very notably from a mere constriction into 

 two, may be briefly recalled. The nucleus of the cell increases in 

 volume, and its scattered chromatin masses group themselves evenly 

 along the linin threads, so that eventually the chromatin seems to 

 be arranged in the form of a long, continuous loop. In the cyto- 

 plasm at one side of the nucleus appears a minute body, the centro- 

 some. This divides into two centrosomes, and they wander apart 

 from each other, each through an angle of 90°, to opposite sides ot 

 the nucleus. These centrosomes are the dynamic centres of the 

 cell division and exert an influence upon the surrounding cyto- 

 plasm, as shown by systems (asters) of cytoplasmic rays converging 

 upon them. Within the nucleus, meanwhile, the chromatin loop 

 has become split through its entire length by an exact halving of 

 each of its larger chromatin masses, and has also broken trans- 

 versely into a fixed number of segments, the chromosomes, which 

 now are .connected together only by linin threads. Then the 

 nuclear membrane dissolves away and a dicentric figure appears 



