232 ESSENTIALS OF BIOLOGY 



and since this substance is repeatedly hydrolyzed by enzymes, or 

 otherwise becomes chemically transformed, it must be the agencies 

 that reassemble, or resynthesize the chromosomes that are the 

 continuously existing things. 



We assume that there are A^ chromosomes in the gonidial cells. 

 Now such a cell is a direct descendant of the fertilized ovum, 

 which has half (or J A^) chromosomes that have been derived from 

 the male parent and J A^ from the female one. In an imaginary 

 case let us say that the gonidial cell, about to mature, has 8 chromo- 

 somes and that half of these, A B C D, are of paternal, and half, 

 abed, are of maternal origin. The gonidial nucleus, then, 

 contains the chromosomes, ABCDabcd, =N. Now, in 

 all the cell-divisions between the phase of the fertilized ovum 

 and the gonidial cell, and in all the cell-divisions by which one 

 gonidial cell becomes many gonidial cells, the chromosomes are 

 always halved and so we conclude that every gonidial cell about 

 to mature has equal numbers of chromatic units derived from 

 paternal and maternal parents. 



There is evidence that either the paternal group, A B C D, 

 or the maternal group, a b c d, is 3. competent agency in the 

 development of the Mendelian characters. (It is true that we 

 have to postulate an ensemble of agencies, or an agency, E, to 

 explain the development of the specific characters, but we do 

 not consider this ensemble here.) When A B C Dis the develop- 

 mental agency (with regard to the " loose " Mendelian characters) 

 the paternal characters are reproduced : conversely abed 

 reproduce the maternal characters. There is evidence that if one 

 chromosome is missing (say there is only A B C, or a b c) the 

 development will be imperfect. It is assumed that whatever A 

 does a will do much the same and so on (except that A gives the 

 paternal bias and a the maternal one). So a normal embryo will 

 come from AbCD, abCD, aBeD and so on (though there 

 will be mixtures of paternal and maternal characters). We may 

 call A and a, B and b, etc., " homologous " chromosomes. We 

 may further call " ^," " 5," " C," " Z) " a " developmental 

 outfit," where '' A '' may be either A or a, '' B " may be either 

 B or b and so on. 



Now when maturation occurs A^ chromosomes are reduced 

 in number to ^ N and this is assumed to be the result of the 

 coupling, in pairs, of the A^ bodies. This coupling is believed 



