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SCIENCE 



[N. S. Vol. XXXVII. No. 961 



of synapsis, and may even be seen more or 

 less distinctly throughout the life of the 

 organism. 



As has been said, the remarkable paral- 

 lel between chromosomes and unit-char- 

 acters constitutes in itself strong (though 

 indirect) evidence that the latter depend 

 in some way upon the former. Specific ex- 

 perimental evidence directly demonstrates 

 the correctness of this conclusion. If, for 

 instance, the orderly distribution of the 

 chromosomes in the fertilized egg is arti- 

 ficially interfered with (as may be done in 

 several ways) the development of the em- 

 bryo is correspondingly disturbed. Boveri 

 has proved that when abnormal combina- 

 tions of the chromosomes are thus pro- 

 duced in the fertilized eggs of sea-urchins 

 the offspring are almost always abnormal, 

 deformed or monstrous. Recent experi- 

 mental studies have proved by various 

 methods that certain interesting abnormal- 

 ities shown in hybrids are preceded by cor- 

 responding disturbances in the chromo- 

 somes. Again, it is now possible to fertil- 

 ize the eggs of such animals as sea-urchins 

 by the spermatozoa of animals as widely 

 different as worms or mollusks. The off- 

 spring of such "heterogeneous" crosses 

 show only the characters of the mother. 

 They are typical sea-urchin larvae ; and the 

 explanation, demonstrated by microscop- 

 ical observation, is that only the chromo- 

 somes of the mother are able to survive in 

 the fertilized egg. Those of the foreign 

 father {i. e., of the sperm cell) sooner or 

 later perish and degenerate within the egg. 



Still another fact, of the same unmis- 

 takable import, is the recently demon- 

 strated relation between the chromosomes 

 and sex. Sex is now definitely known to 

 be inherited like other characters; and 

 within a few years the decisive proof has 

 been attained that the heredity of sex is 

 connected with a particular chromosome 



known as the "sex-chromosome" or "X- 

 chromosome. " In a large class of cases, to 

 which man almost certainly belongs, the 

 male contains but one of these chromo- 

 somes, the female two; hence in such cases 

 the total number of chromosomes in the 

 female is one greater than in the male. In 

 respect to these particular chromosomes, 

 accordingly, the male always remains of 

 simplex composition (XO), while the fe- 

 male is of duplex (XX). Observation has 

 proved further than when the duplex 

 chromosome-group of the female are re- 

 duced to simplex ones each mature egg re- 

 tains a single X-chromosome, while in the 

 male only half the spermatozoa receive X 

 and half do not. From this it follows 

 that when the egg (X) is fertilized by a 

 sperm-cell containing X the result is a fe- 

 male (XX), while if fertilized by a sperm- 

 cell without X, the result is a male (XO). 

 I shall try to show a little later how clearly 

 and simply these facts explain certain very 

 curious special phenomena connected with 

 the heredity of sex. 



The specific and direct evidence thus 

 briefly outlined has definitely established 

 the fact that the chromosomes are causal 

 agents in heredity; and it has already be- 

 come evident that the study of their modes 

 of distribution, combination and recombi- 

 nation provides us with a key which will 

 unlock many special puzzles of heredity 

 which would otherwise seem to us insoluble. 

 I will attempt to make this clear in greater 

 detail by considering three of the particu- 

 lar cases that have already been touched 

 upon, taking them up in the order of their 

 difficulty. 



The simplest of these cases is that of re- 

 version, illustrated by the sweet peas. It 

 is evident from the experimental results 

 that the purple color of the flowers requires 

 the cooperation of at least two things, 

 either of which alone is unable to produce 



