3i6 EVOLUTION, GENETICS, AND EUGENICS 



which keeps the chromosomes of constant size, shape and number; 

 which partitions them so accurately in ordinary cell-division; 

 and which provides for a reduction of their numbers by half in the 

 germ-cell while yet securing that each mature gamete gets one of each 

 kind of chromosome. Most biologists look on these facts as indicating 

 that the chromosomes are specifically concerned in inheritance. 



In the first place it is recognized that as regards the definable 

 characters which separate individuals of the same species, offspring 

 may inherit equally from either parent. And it is a very significant 

 fact that while the ovum and spermatozoon are very unequal in size 

 themselves, the chromosomes of the two germ-cells are of the same 

 size and number. This parity in chromosomal contribution points 

 clearly to the means by which an equal number of character deter- 

 miners might be conveyed from each parent. Moreover it is mainly 

 the nucleus of the sperm-cell in some organisms which enters the egg, 

 hence the determiners from the male lin : must exist wholly or largely 

 somewhere in the nucleus. And the bulk of the nucleus in the sper- 

 matozoon consists of the chromosomes or their products. 



A single set of chromosomes derived from one parent only is 

 sufficient for the production of a complete organism. — That a single 

 or haploid set of chromosomes as seen in the gametes is suflicient 

 contribution of chromatin for the production of a complete organism 

 is proved by the fact that the unfertilized eggs of various animals 

 (many echinoderms, worms, mollusks, and even the frog) may be 

 artificially stimulated to development without uniting at all with a 

 spermatozoon. The resulting individual is normal in every respect 

 except that instead of the usual diploid number it has only the single 

 or haploid number of chromosomes. Its inheritance of course is 

 whoUy of maternal origin. The converse experiment in echinoderms 

 in which a nucleus of male origin (that is, a spermatozoon) has been 

 introduced into an egg from which the original nucleus has been 

 removed shows that the single set of chromosomes carried by the male 

 gamete is also sufficient to cooperate with the egg-cytoplasm in 

 developing a complete individual. 



The duality of the body and the singleness of the germ. — Since 

 every maternal chromosome in the ordinary cell has an equivalent 

 mate derived from the male parent, it follows therefore, supposing the 

 chromosomes do have the significance in inheritance attributed to 

 them, that as regards the measurable inheritable differences between 

 two individuals, the ordinary organism produced through the union 



