REPRODUCTION. 647 



cell, and no group of cells used for the physiological mainte- 

 nance, of the individual, can build up a new complete living 

 being; but the continuance of the race is specially provided 

 for by setting apart certain cells which shall have this one 

 property cells whose duty is to the species and not to any one 

 representative of it an essentially altruistic element in the 

 otherwise egoistic whole. 



Sexual Reproduction. In some cases, especially among 

 insects, the specialized reproductive cells can develop, each for 

 itself, under suitable conditions, and give rise to new indi- 

 viduals; such a mode of reproduction is called parthenogenesis: 

 but in the majority of cases, and always in the higher animals, 

 this is not so; the fusion of two cells, or of products of two 

 cells, is a necessary preliminary to development. Commonly 

 the coalescing cells differ considerably in size and form, and 

 one takes a more direct share in the developmental processes; 

 this is the egg-cell or ovum; the other is the sperm-cell or 

 spermatozoon. The fusion of the two is known as fertiliza- 

 tion. Animals producing both ova and spermatozoa are 

 hermaphrodite; those bearing ova only, female; and those 

 spermatozoa only, male : hermaphrodiBsm is not found in 

 Vertebrates, except in rare and doubtful cases of monstrosity. 



Accessory Reproductive Organs. The organ in which 

 ova are produced is known as the ovary, that forming sperma- 

 tozoa, as the testis or testicle; but in different groups of animals 

 many additional accessory parts may be developed. Thus, 

 in all but the very lowest Mammalia, the offspring is nourished 

 for a considerable portion of its early life within the body of 

 its mother, a special cavity, the uterus or womb, being pro- 

 vided for this purpose: the womb communicates with the 

 exterior by a passage, the vagina; and two tubes, the oviducts 

 or Fallopian tubes, convey the eggs to it from the ovaries. 

 In addition, mammary glands provide milk for the nourish- 

 ment of the young in the first months after birth. In the 

 male mammal we find as accessory reproductive organs, vasa 

 defer entia which convey from the testes the seminal fluid con- 

 taining spermatozoa; vesiculce seminales (not present in all 

 Mammalia) , glands whose secretion is mixed with that of the 

 testes or is expelled after it in the sexual act ; a prostate gland, 

 whose secretion is added to the semen; and an erectile organ, 

 the penis, by which the fertilizing liquid is conveyed into the 

 vagina of the female. 



