REPRODUCTION 5 I 



are less marked, the tail and sometimes also the gills being retained. 

 In some urodeles the changes are comparatively insignificant and such 

 salamanders as adult Necturus are often referred to as "permanent 

 larvae." 



The duration of the larval period ranges from a few weeks in some 

 salamanders to a year and more in some frogs. The amount of growth 

 attained during the period varies accordingly. Sexual maturity is ordi- 

 narily not attained during the larval period although exceptionally it may 

 be. The Mexican axolotyl, the larva of the salamander, Ambystoma 

 tigrinum, regularly breeds in the larval state. 



It is evident, then, that in the development of most amphibians the 

 all-important source of food is the yolk deposited in the egg. This carries 

 development on to the beginning of the larval period or, in fact, the animal 

 may enter the larval period with an important remnant of the yolk still 

 available for further growth. If the larval stage is passed in the open 

 water, the tadpole proceeds to feed, thus augmenting the original food 

 supply. Given a prolonged larval period, a relatively great size may be 

 attained before metamorphosis. When the larval period is spent within 

 the oviduct, or within a dermal sac or a vocal pouch, there is obviously no 

 chance of feeding and less growth is possible before metamorphosis. 

 It should be noted, however, that the extraordinary development of the 

 gills in some of these confined tadpoles opens the possibility that some 

 nutriment as well as oxygen may be absorbed from the surrounding 

 maternal, or sometimes paternal, tissues or fluids. For example, the 

 tadpoles of the "marsupial" frog have enormously expanded bell-shaped 

 gills which expose a large and highly vascular larval surface to the interior 

 of the marsupial pouch. Such a surface might easily serve for absorption 

 of nutrient material as well as oxygen. 



Prior to the development of the larval gills the respiration of the embryo 

 must be carried on by diffusion through the general outer surfaces whether 

 the environment be water or some contiguous parental tissues and fluids. 



In Reptiles and Birds. In striking contrast to the reproductive 

 arrangements in fishes and amphibians, where the utmost diversity pre- 

 vails, the corresponding arrangements in reptiles and birds are marked 

 by a high degree of uniformity. The egg, always very large in proportion 

 to the size of the animal, contains a quantity of yolk sufficient to provide 

 for the whole developmental period, thus enabling the young animal to 

 attain a relatively large size (in contrast to the corresponding conditions 

 exhibited by most fishes and amphibians) before being cast upon its own 

 resources for obtaining food. A newly-hatched alligator is gigantic com- 

 pared to a newly-hatched salmon. 



All birds and most reptiles are oviparous. Immediate protection for 

 the egg is provided by the strongly developed shell. Reptiles deposit the 



