56 COMPARATIVE ANATOMY 



the net results are equally good, or nearly so, and generation after genera- 

 tion the life of the world goes on with, at most, only very slow change in 

 the general biological balance and scheme of things. 



Among vertebrates may be recognized four fairly sharply defined 

 complexes of reproductive conditions. They may be briefly summarized 

 as follows: 



I. Eggs small: very little yolk 



1. As in many Teleostei 

 Oviparity 



Very many eggs produced — thousands or millions 



No parental care 



Rapid development 



Early differentiation 



Early self-supporting 



Long growth period after hatching 



High mortality 



2. As in higher mammals: minimum yolk 

 Viviparity 



Few eggs produced 

 Placental development 

 Differentiation mainly pre-natal 

 Growth largely pre-natal 

 Post-natal supplementary nutrition 

 Parental care 

 Self-support deferred 

 Maximum survival 

 II. Eggs of medium size (1.5-6 mm.): moderate amount of yolk 



3. As in many amphibians, especially .\nura 

 Oviparity usual 



Moderate number of eggs — usually scores or hundreds 

 Parental protection in many cases 

 Free-living self-supporting larva 

 Much growth during larval stage 

 Metamorphosis 

 Considerable mortality 

 III. Eggs very large: much yolk 



4. As in reptiles and birds 

 Oviparity usual 



Eggs few — necessarily so because of size 



Protective shell 



Amnion and allantois 



More or less parental care 



Differentiation mainly before hatching 



Growth largely before hatching 



Self-support immediately or soon after hatching 



Low mortality 



There are, as already set forth, a great many modifications of these 

 four types of reproduction. 



