PHYSIOLOGY 193 



the same time. Under the section on reproduction in starfish such 

 a procedure is described. In animals like the toads and frogs, 

 a special provision is made to bring the individuals of the op- 

 posite sexes together in that the male clasps the female and 

 sheds sperm over the eggs as they are expelled from the cloaca. 

 This act is known as amphiplexus. The first and second pairs 

 of abdominal appendages of the male crayfish are modified for trans- 

 ferring spermatozoa into the seminal receptacle of the female, where 

 they remain until the eggs are laid. This represents a beginning in 

 the development of a copulatory organ. The majority of bisexual 

 or dioecious animals make a still greater provision to insure fertili- 

 zation of the ova by copulation or coitus. At the time of breeding 

 the mature spermatozoa are delivered to the cloaca or vagina of the 

 female, and the ova are fertilized within the urinogenital tract of the 

 female. 



In birds and reptiles after the addition of nutritive and protective 

 coats the egg passes to the outside to develop and hatch (oviparous 

 animals) but in all mammals, except monotremes, it is retained 

 within the uterus during the period of embryonic development, and 

 the young are born as more or less developed individuals (vivipa- 

 rous). In the females of viviparous mammals the posterior portions 

 of the two oviducts are modified into a uterus within which the young 

 are retained and nourished until ready for birth. The internal wall 

 of the uterus and the external embryonic membranes (serosa and 

 allantois-chorion) cooperate to form a placenta through which food, 

 metabolic wastes, and respiratory gases diffuse between parental and 

 embryonic blood. The blood does not pass from parent to embryo or 

 vice versa but the necessary materials are allowed to diffuse through 

 the tissue of the placenta in which both systems are distributed. 



Parthenogenesis.- — In some species of invertebrates, sexual repro- 

 duction may lapse for considerable periods of time, during which 

 period no males are developed. The female produces ova which 

 develop into new individuals like herself without fertilization for 

 a whole season. This is known as partheyiogenesis. Usually in the 

 fall of the year males are developed, and fertile eggs, provided with 

 protective hard shells, are produced by the females of this generation 

 to live through the winter. After winter is over such fertile eggs 

 hatch into parthenogenetic females for the next season. This process 

 is common in many smaller Crustacea, aphids, scale insects, some ants, 

 bees, wasps, thrips, a few moths, and rotifers. Artificial partheno- 



