MEANS AND METHODS OF REPRODUCTION 161 



the greatest demand arises in connection with breeding activities. 

 Once these are completed, fat begins to accumulate in the fat 

 bodies until by autumn they are restored to the maximum size. 



Amplexus. — In amplexus the male embraces the female from 

 the dorsal side, clamping its forelegs about the female just 

 behind the latter's forelegs, firmly gripping the body of the 

 female with the enlarged sexual pad of the first digit of each 

 hand. Captured female frogs may lay eggs in the laboratory in 

 the absence of males, but under natural conditions amplexus 

 acts as a stimulus to spawning. Amplexus takes place in the 

 water. Under its stimulus the female sheds its eggs and as the 

 latter pass from the cloaca, the male emits sperm over them. 

 The amplectant posture persists for three or four days until all 

 the eggs are shed. 



When the eggs leave the female cloaca, the thickness of the 

 capsule of the surrounding jelly is less than one-half the diameter 

 of the egg. A single spermatozoon now enters each egg, after 

 which the capsule absorbs water and swells to two or three times 

 its original thickness. At this time it can be readily seen that 

 the capsule is really composed of three layers. The gelatinous 

 capsule is comparable to the white of the hen's egg and serves as 

 a protective covering. The jelly also serves another function in 

 retaining heat absorbed from sunlight. The jelly seems to 

 transmit the shorter rays but checks the loss by radiation of the 

 longer heat waves from the egg. 



Intrauterine Development. — The young of viviparous ani- 

 mals, whether invertebrates or vertebrates, are brought forth in 

 a more or less fully developed state. What happens in the 

 majority of cases, among invertebrates and among vertebrates 

 below Mammalia, is that the large-yolked egg after fertilization 

 remains in a specialized portion of the oviduct, known as the 

 uterus, and develop there instead of outside the body. Thus in 

 viviparous fishes and snakes, the yolk of the egg seems to be the 

 main source of nutrition for the embryo, though an organic 

 attachment of the egg to the wall of the uterus may occur in 

 some cases, as in Mustelus, a common viviparous shark (Fig. 107). 

 All mammals are viviparous with the exception of the lowest 

 group, the Prototheria, which are oviparous. The uterus of 

 viviparous mammals is a modified region of the oviduct whose 

 walls are highly vascular and come into close enough contact 



