THE REPRODUCTION OF ANIMALS 227 



tion is required, but, in addition, the male now actively seeks the female 

 and clasps her or remains in very close proximity while eggs and sperm 

 are discharged into the same very limited area, so that the eggs emerge 

 from the female into a swarm of spermatozoa. A wide variation in details 

 of this method is shown by the frogs, toads, and most of our fresh-water 

 fishes. 



Internal Fertilization without Copulation. Internal fertilization in- 

 volves the liberation of the spermatozoa within the reproductive tract 

 of the female. Here the sperms find a fluid medium in which to swim, 

 and their path is so limited and directed that they are almost certain to 

 encounter any eggs that are present. In a few forms, particularly some 

 of the tailed amphibians, internal fertilization is accomplished by the 

 males depositing sperm-filled capsules that the females find and take 

 into their reproductive tracts. In some forms this is done without the 

 males and females coming into direct contact with one another; in others, 

 the sperm-filled capsule (spermatophore) is transferred from male to 

 female during a behavior that involves amplexus but not copulation. 



Internal Fertilization with Copulation. In nearly all other forms in- 

 ternal fertilization is accomplished by copulation; i.e., by the direct trans- 

 ference of spermatozoa into the reproductive tract of the female by some 

 intromittent organ (the penis, in the case of mammals) that forms a part 

 of the male's accessory sex apparatus. In nearly all such forms the actual 

 copulation constitutes but a small part of the complicated courtship and 

 reproductive behavior that culminates in fertilization. 



Care of the fertilized eggs and young. In many animals the pecul- 

 iar adaptive breeding behavior is continued into nesting habits and 

 various types of care for the young. Once the egg is fertilized, develop- 

 ment begins. The materials and energy required for at least the early 

 stages of development either are supplied by food contained in the egg 

 (stored there before the egg left the mother's body) or are furnished 

 by the mother as development proceeds. In the former case the egg is 

 usually "laid," and we speak of such a habit as oviparous. Birds, amphib- 

 ians, and most reptiles, fish, insects, and lower animals furnish examples 

 of an oviparous habit. Among oviparous animals the amount of food 

 that is stored in the egg varies widely. At one extreme we have such eggs 

 as those of the starfish, which contain very little food material (yolk). 

 Such eggs contain too little food to carry development very far, and the 

 young are hatched in a very primitive (larval) condition that is quite 

 unlike the parent form. Other oviparous eggs contain much food material, 

 stored in one part of the egg. Frogs, fish, birds, and reptiles, as well as 

 the arthropods and most of the mollusks, produce eggs of this kind. In 

 the frog's egg the yolk forms little more than half the bulk of the egg; in 

 the fish's egg the yolk forms a much larger proportion ; and in the eggs of 



