REPRODUCTION AND LIFE CYCLES 113 



however, cross-fertilization occurs, since the ova and spermatozoa 

 mature at different times. Reported cases of functional hermaphro- 

 ditism among mammals appear to be highly doubtful. 



Parthenogenesis 



The development of an egg without fertilization by a sperm occurs 

 quite commonly under natural conditions in some invertebrate forms. 

 Usually there is a cessation of activity on the part of the males for a 

 period of time when ova, produced by the females, develop into 

 apparently normal individuals. In some few instances males are 

 permanently absent. The rotifers, water fleas (Cladocera), and plant 

 hce (aphids) all exhibit this type of development at times. In cladoc- 

 era, of which Daphnia is a well-known example, the females produce 

 parthenogenetic eggs during the warm weather. From two to twenty 

 eggs, depending upon the species, are deposited and nourished in the 

 brood-sac. Usually several generations of females will be produced 

 in this fashion. Eventually male as well as female daphnids are 

 produced, and the eggs from this generation of females must be 

 fertilized by the males. When fertilization occurs, the eggs are 

 covered by the highly resistant protective portion of the brood-sac 

 (ephippium) which enables them to withstand desiccation and the 

 rigors of winter. 



Numerous experimenters have been interested in attempts to induce 

 artificial parthenogenesis in various invertebrate eggs by means of 

 chemical or physical stimuli ranging all the way from simple salts 

 and complex fatty acids to mechanical means, such as pricking with a 

 needle, shaking, or raising the temperature of the water surrounding 

 the experimental organisms. Mead first successfully induced arti- 

 ficial parthenogenesis with the ova of annelids and Loeb extended 

 the experiments to include starfishes, sea urchins, molluscs, and even 

 frogs, which underwent at least partial development by means of 

 various chemical or physical stimuli aptly described as parthenoge- 

 netic agents. 



Most of the experimental efforts to induce parthenogenesis in 

 vertebrates have been rewarded by failure. In a few instances tad- 

 poles have been produced through mechanically initiating cleavage 

 of the egg by pricking with a needle and introducing a small amount of 

 blood serum at the same time. Pincus has also been able to carry a 

 mammal embryo through early developmental stages after partheno- 

 genetic stimulation. 



