Xaitre of Heredity. 3-7 



This belief is strf)n-;lv su])iH)iie(l by ihe phenomena of cell-division, 

 in which the nucleus invariablv takes the lead, and the general proto- 

 plasm of the cell follows afterwards. The unicellular Aimcba, 

 for example, consisting of a but slightly difTerentiated mass of cell- 

 protoplasm {Cyioplasni). with a nucleus near the middle, reproduces 

 itself bv such cell-division. When the organism has attained a cer- 

 tain size the nucleus, consisting in large part of a .special kind ot 

 protoplasm distinguished as the chromatin substance, divides inio iu(. 

 parts which move awa\ from one another. The surrounding 

 cvtoplasm then contracts into a narrow bridge l)etween the i\vi> 

 nuclei, and finally breaks across, and the division of the cell, in this 

 case also an entire organism, is complete. 



It has been shewn experimentally that an Anucba may also In- 

 made to mutiply artificially by cutting it in pieces, and it is a fact 

 of the highest significance with regard to the importance of the 

 nucleus that it is Duly when a part of the nucleus is present in it that 

 any particular fragment will live and grow. 



In the developing egg of the higher animals and plants, again. 

 the nucleus invariably takes the initiative, and the cell-protoplasm 

 follows. Not only is this the case, but it has been shown experi- 

 mentallv. as Professor Minot ol)serves. that in the Frog"s egg " the 

 jiosition of the nucleus determines which part of the ovum shall 

 become the dorsal surface of the emfirvo." 



The structure of the cell-nucleus is. in reality, extremely com- 

 plex, far more so than that of the surrounding cytoplasm, and in ihe 

 vast majoritv of cases of cell-division a veri" elaborate re-arrange- 

 ment of its constituent parts, known as " karyokincsis." may be 

 observed. In this process certain parts of the nucleus known as the 

 cliroinosoiuss, v.hich have the form of longer or sht^rter threads, 

 plav a verv important part, and though we b\ no means understand 

 the full meaning of the })henomena in question, we are tolerably safe 

 in believing with Weismami that one result of the karyokinetic 

 process is to secure a ver\ accurate qualitative as well as a quantita- 

 tive division of the chromatin substance (chromosomes). During the 

 process definite centres of force are established at opposite poles of the 

 nucleus, to which the halves of the accuratelv divided chromo.somes 

 are attracted, one half of each chromosome being pulled towards each 

 pole ill a manner highl\ suggestive of magnetic attraction. In this 

 process of nuclear division the membrane in which the nucleus is 

 normally enclosed disappears, so that the general cell-protoplasm 

 (cytoplasm) is no longer sharply divided from the nucleoplasm 

 between the chromosomes, while the common protoplasmic mass thus 

 formed exhibits strong lines of radiation around the polar " spheres 

 of attraction." Ultimately the general protoplasm of the cell divides 

 into two parts, which arrange themselves around the two groups of 

 chromosomes, and the cytoplasm is again shut ofif from the nucleo- 

 plasm bv the formation of a new nuclear membrane. 



The almost if not quite universal occurrence of a nucleus, or at 

 anv rate of chromatin substance, within the cell-bodv. even in the 



