STRUCTURE AND DEVELOPMENTAL RATE 121 



moisture, oxygen supply, and temperature are comparatively 

 uniform, and although the eggs may develop faster or slower 

 under slightly different conditions of temperature, etc., yet the 

 variations in the medium are rarely sufficient to inhibit or stop 

 development entirely, and when they are the eggs usually die. 



On leaving the sea the fresh-water and land-living inverte- 

 brates and vertebrates show most varied and complex methods 

 and arrangements for insuring an environment of sufficient uni- 

 formity to permit an uninterrupted development. Many forms, 

 as is also the case in certain sea-living animals, have evolved 

 a method for the development of the embryo within the body of 

 the mother. Such an internal environment tends to control very 

 effectively the conditions of moisture and in mammals also the 

 temperature, but at times, as we shall see beyond, the oxygen sup- 

 ply is not properly adjusted and the continuity of development 

 may be interrupted or interfered with on this account. 



The land-living animals have not always succeeded in obtain- 

 ing an ideal developmental environment, and there are many 

 examples of a discontinuous mode of development as a result of 

 environmental breaks in the strictest sense. That is, the egg 

 begins to develop and attains a certain stage, when a more or 

 less sudden change or break in the environment occurs and devel- 

 opment stops completely and may remain at a standstill for vari- 

 ous lengths of time — days or possibly weeks. Another alteration 

 in the environment then occurs which again permits development 

 to start and continue until the fully formed animal is obtained. 

 Such a discontinuous mode of development is universal among 

 one great class of vertebrates, the birds. Among the birds devel- 

 opment, as far as studied, is invariably interrupted when about 

 the stage' of gastrulation, at which time the egg is laid or passed 

 out of the warm body of the mother. The fall in temperature 

 causes development to stop and the egg remains in the gastrular 

 stage until incubated by the heat of the parent's body or until 

 artificially incubated at a similar temperature. 



The means of interrupting development seem to reside entirely 

 outside the egg itself, they are properties of the environment. 

 As far as is known, all eggs having once begun to develop will pro- 



