THE VERTEBRATE ANIMAL — SUBPHYLUM VERTEBRATA 409 



other hermaphroditic forms (as some cyclostomes) the spermatozoa 

 and ova of a particular individual are usually not mature at the 

 same time. 



Bisexual reproduction is the form of reproduction common to 

 many groups of the higher invertebrates and nearly all vertebrates. 

 Here the sexes are distinct, each with functional gonads abd ac- 

 cessory structures capable of producing only one kind of germ cells. 

 In some of the types of animals already studied individuals of the 

 two sexes have simply deposited the mature germ cells in the same 

 vicinity and at about the same time. Under the sections on re- 

 production in starfish and the bullhead (fish) such a procedure has 

 been described. In animals like the toads and frogs, a special pro- 

 vision is made to bring the individuals of the opposite sexes to- 

 gether 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. It will be remembered that 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 tha 

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

 female. 



In birds and most reptiles after the addition of nutritive and pro- 

 tective coats the egg passes to the outside to develop and hatch (ovip- 

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

 within the uterus during the period of embryonic development, and 

 the young are bom 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 



