No. 464.] STUDIES ON PLANT CELL — VII. 



573 



of others, who have confirmed his conclusions, is the discovery 

 in a number of animals and plants that the germ cells of the 

 hybrid may be pure with respect to certain characters of the 

 parents which are crossed. This principle is not without excep- 

 tions where the conditions are apparently complicated by unusual 

 factors but the phenomenon when present is so striking as to 

 command immediate attention and call for an explanation on a 

 cytological basis. The purity of the germ cells of hybrids means 

 in the words of Castle that " the hybrid, whatever its own char- 

 acter, produces ripe germ cells which bear only the pure char- 

 acters of one parent or the other." Thus if two forms A and B 

 are crossed the hybrid will have embodied in itself the characters 

 AB, one of which however may lie latent, i. e., may not be visi- 

 ble in the hybrid. Such a latent character when present is 

 termed recessive while the prominent character is termed domi- 

 nant. In a simple case some of the offspring of the hybrid AB 

 will be found to have the character of A alone, some of them of 

 B alone, and some of them will again have the mixed characters 

 AB. If experiments are carried out on an extensive scale the 

 proportions of these offspring from the hybrid may exhibit the 

 remarkable fact that there are about twice as many forms of AB 

 as either A or B, i. e., the proportions of A's, AB's, and B's are 

 in the ratio of 1:2:1. Furthermore the offspring of A when 

 bred among themselves remain absolutely true producing a suc- 

 cession of pure forms all A's and the same results follow when 

 the offspring of B are closely bred. But when forms with the 

 mixed" characters AB are bred with one another their offspring 

 break up as before into three types A, AB, and B in numerical 

 proportions expressed by the same ratio 1:2:1. The history 

 is simply told in the following diagram where the number of off- 

 spring is assumed to be 4. 



Form A^ 



I A. 



: Hybrid AB-^i 2AB< 



Form B^ 



.4A. 

 2 A. 



16A. 

 8A. 

 4A. 



..64A 

 . .32 A 

 ..16 A 

 f 8A 



4AB<| 8AB.{ 16AB 



2B. 

 .4B. 



4B. 



8B. 



16B. 



8B 

 16B 

 32 B 

 64 B 



