214 GENERAL BIOLOGY 



taining deep cavities next the respiratory epithelium, where 

 a zone of moisture -laden residual air serves as a medium of 

 exchange and as a buffer to the air waves from the outside. 



The amphibians make it easy to understand the transition 

 from aquatic to terrestrial life in vertebrates. It would 

 have been hard to imagine all the changes necessary for 

 fitting a fish-like aquatic vertebrate for life on land, but in 

 a salamander these changes, some of which would certainly 

 surpass imagining, are all wrought out in a little while 

 before our eyes; they go forward without a hitch, and 

 most significant of all, they go forward in similar manner, 

 in all the higher terrestrial A^ertebrates, whether they are to 

 live any part of their lives in the water or not. 



As in the salamander, so in vertebrates generally, the sexes 

 are separate and true sexual reproduction is universal. 

 But there is very great diversity among them as to mode 

 of nurture of young, and some of the differences are of 

 profound significance. The lancelet (fig. 132) lays minute 

 eggs containing very little yolk; these segment and gas- 

 trulate typically, and the embryos hatch when they reach 

 the gastrula stage, and thereafter shift for themselves, 

 receiving no further parental nurture. But all the domi- 

 nant groups of vertebrates make better provision for the 

 development of their offspring, and do not turn them 

 adrift in so immature and feeble and defenseless a con- 

 dition. 



Types of nurture. — There are two main types of nurture 

 for the young of vertebrates, i) The storing of additional 

 food supply in the form of yolk in the eggs. We have found 

 a considerable store of yolk in the eggs of salamander; 

 this process reaches its maximum development in the 

 relatively huge eggs of birds. 



2) The nurture of the young by means of embryonic 

 membranes. This reaches its maximum development in 



