1122 THE REPRODUCTIVE ORGANS 



On crossing a plant of a tall variety with one of a dwarf type, he 

 noted that the seeds obtained from them gave rise exclusively to tall 

 plants. When the latter were then recrossed among themselves, the 

 result was 75 per cent, of tall and 25 per cent, of dwarf plants. The 

 subsequent crossing of the latter with one another yielded only dwarf 

 plants through successive generations. The former, on the other hand, 

 fell into two groups, because while 25 per cent, of them continued to 

 yield tall types, the other 50 per cent, gave rise to 75 per cent, of tall and 

 25 per cent, of dwarf plants. Mendel explained these results by stating 

 that characters are either dominant or recessive. In the preceding ex- 

 ample, the tallness is dominant and the dwarf condition recessant. 



This principle may be made clearer at the hand of the following example: If 

 a gray (A) and white (B) mouse are crossed, the offspring (Ci) will be all gray. 

 If the gray mice are now bred to each other, the young (C 2 ) will be either gray or 

 white in the proportion of 3:1. On crossing the white of this generation, only 

 white offspring will be obtained throughout. The gray individuals, on the other 

 hand, will give rise to one-third of gray and two-thirds of white offsprings. On 

 recrossing the former only gray young are gotten, while the latter yield both white 

 and gray. From the white of this last generation only pure white are obtained, 

 while the gray may be either pure gray or gray-dominant-white recessives A (B). 



In applying Mendel's Law to animals, it may be assumed that the 

 two kinds of germ-cells are individualized as dominants and recessants 

 or that the germ-cells of the hybrid are alike, i.e., that they contain 

 both dominant and recessive characters which are then either brought 

 forth or suppressed during fertilization in consequence of certain 

 external and internal factors. While the former explanation is the 

 more simple, it nevertheless fails to account for certain phenomena, 

 as does, in fact, Mendel's Law itself. Thus, the crossing between 

 members of the white and black races of man does not give rise to 

 either type, but to an intermediate progeny, showing various degrees 

 of admixture. 



CHAPTER XCIV 



THE MALE AND FEMALE REPRODUCTIVE ORGANS 



The Testicles. The reproductive organs of the higher animals 

 may be divided into two classes, namely, those actually producing the 

 generative elements, and those serving as a means of bringing these two 

 elements together. The former embrace the testes and ovaries, and the 

 latter, the penis, seminal ducts, vagina, uterus and Fallopian tubes. 

 The essential sexual organ of the male comprises the testes, two oblong 

 glands, each of which measures about 4 cm. in length, 3 cm. in breadth 

 and 2 cm. in thickness it weighs 15 to 25 grams. These organs are 

 contained in a sac-like appendage, the scrotum, which is divided into two 



