Chap. 18 REPRODUCTION 339 



and acts upon the ovaries. The twin heifer is never sterile unless the mem- 

 branes of the male and female embryos are fused. Freemartins are known 

 to occur only in cattle, pigs, and goats. 



Intersexes. Any normal, sexually produced animal has some structures of 

 the opposite sex. Intersexes are individuals in which the development of such 

 structures is carried to a more or less marked degree, actually degrees of 

 hermaphroditism. Many examples of intersexes show that the plans of the 

 male and female bodies are fundamentally similar and delicately balanced. A 

 tilt in one direction lifts the maleness, in the other the femaleness. 



Parthenogenesis. Eggs may develop without fertilization, i.e., partheno- 

 genetically. Natural parthenogenesis is known only in the invertebrates, 

 notably in many small crustaceans and certain orders of insects. In social 

 ants, bees, and wasps the queen can lay either fertilized or unfertilized eggs. 

 Male honeybees develop parthenogenetically. In most aphids (plant lice), 

 there is one generation after another of wingless females, great populations 

 in which every individual produces young from unfertilized eggs. In autumn, 

 these are succeeded by a generation of parthenogenetically produced winged 

 males and females from which fertilized, winter-hardy eggs are produced. 

 In the spring a generation of females hatches from these and the program 

 of the previous summer is repeated. Again every plant louse is busy on the 

 production line; each one means dozens more. 



Artificial Parthenogenesis. Certain kinds of eggs that normally re- 

 quire fertilization can be stimulated artificially by chemical and physical 

 means to develop into embryos; some even grow into adult animals. This 

 can be done by jolting them in revolving egg-shakers, pricking them with a 

 fine needle, raising the temperature, or changing the content of the fluid 

 about them. Eggs of cold-blooded animals that lay their eggs in open water 

 were the first to be tested, those of starfishes, sea-urchins, and frogs being 

 easiest to manipulate. The possibilities of artificial parthenogenesis were 

 first clearly demonstrated in 1900 by Jacques Loeb who obtained over 200 

 tadpoles by stimulating frogs' eggs. About half of these lived to become 

 well-grown young frogs of both sexes, the famous "fatherless frogs." When 

 their tissues were examined, the cells of most of these proved to have the 

 usual double number of chromosomes characteristic of frogs of their species 

 and not half that number as might have been expected. More recently, eggs 

 taken from the oviducts of rabbits have been stimulated by changes of 

 fluid and temperature. These cleaving eggs were transplanted into the uteri 

 of unmated foster mothers where some developed normally. After birth, they 

 grew into adults as fatherless as the distinguished frogs of 1900. Such ex- 

 periments show that an egg is capable of developing without the biparental 

 inheritance, and that development may be started by physical or chemical 

 means, possibly stimulating enzymes within the egg that are ordinarily ac- 



