May 30, 1913] 



SCIENCE 



819 



offspring are therefore of double or "du- 

 plex" hereditary constitution. When the 

 germ-cells are formed, as Mendel first 

 proved by experiments on hybrids, they are 

 found to carry only a single or "simplex" 

 group of characters. So closely parallel is 

 all this to what we have learned about the 

 chromosomes that we can just as well use 

 the diagram for the chromosomes as for the 

 characters. Chromosomes and characters 

 alike form a single or simplex group in the 

 germ-cells, a double or duplex group in 

 the body of the offspring ; and this alone is 

 sufficient to make it extremely proballe 

 that chromosomes and characters are some- 

 kow connected. Exactly what is the nature 

 of this connection we are not able to say 

 with certainty; but we might reasonably 

 assume, for instance, that each character 

 depends upon some particular chemical 

 substance, or group of substances, con- 

 tained in the chromosomes, and that dif- 

 ferent chromosomes differ in respect to the 

 substances which they carry. Such an as- 

 sumption would be thoroughly in accord 

 with the prineiplfts of chemical physiology 

 and with the results of experiments upon 

 the physiology of development. 



The diagram brings out another funda- 

 mentally important fact that was also 

 proved by Mendel's experiments, namely, 

 that in the formation of the simplex char- 

 acter-groups all possible recombinations of 

 the original parental unit-characters 

 (within the limits of a single complete 

 group) are effected. Only a few of the 

 germ-cells receive the original combina- 

 tions unchanged (A-D or a-d). In most 

 cases new simplex groups are formed by 

 recombination, such that each germ-cell 

 receives always a complete single series 

 (from A or a to D or d) , but any particular 

 member of the series may be derived from 

 either parent. The number of such pos- 

 sible combinations varies, of course, with 



the simplex number of unit characters; 

 with 4 characters, as in the present ease, 

 it is 16 ; with 15 characters it would be 

 more than thirty thousand. Any individ- 

 ual may thus produce many different kinds 

 of germ-cells, equivalent in a general way 

 but differing slightly in respect to their 

 individual hereditary components. This 

 result follows from the fact, discovered by 

 Mendel again, that corresponding or homo- 

 logous parental components of the duplex 

 groups (such as A and a, or B and b) 

 never enter the same germ-cell; and this 

 is the essential fact in "Mendel's Law." 

 We could readily understand this if before 

 the germ-cells are formed corresponding 

 parental components become associated in 

 pairs (Aa, Bb, etc.) and then separated 

 or disjoined in an ensuing process of 

 division. If the process of disjunction 

 took place in each pair independently of 

 the others, all combinations would obvi- 

 ously be produced in the resulting germ- 

 cells. Now, it is certain that something 

 like this actually takes place in the case of 

 the chromosomes. In the process known as 

 synapsis, which takes place shortly before 

 the last two cell-divisions concerned in the 

 formation of the germ-cells, the chromo- 

 somes do in fact unite in pairs, two by 

 two. There is reason to believe that the 

 two members of each pair are respectively 

 of maternal and paternal derivation ; and 

 the probability of this view, first stated 

 by Montgomery, has steadily increased. 

 Observation has made it extremely prob- 

 able that in the course of the following 

 two divisions the two members of each 

 pair, or two somethings that they con- 

 tain, are separated so as to pass into dif- 

 ferent germ-cells (Fig. 1). One of the 

 most interesting recent discoveries in cytol- 

 ogy is the fact that in some animals and 

 plants a paired arrangement of the chro- 

 mosomes is assumed long before the period 



