120 CHARLES R. STOCKARD 



Although there is a definitely normal rate of development for 

 a given egg, this rate is frequently subject to wide variations, usu- 

 ally as a result of variations in the surrounding conditions. The 

 two chief, or most frequent, modifying causes are a change in 

 oxygen supply or a change in temperature. An acceleration of 

 the usual rate only takes place to a limited degree under natural 

 conditions and but slight increases in developmental rate have 

 been experimentally obtained. On the other hand, a very wide 

 range of decrease in developmental rate is readily brought about. 

 Slight changes in the surrounding temperature or reduction in the 

 oxygen supply will readily tend to slow the rate of development 

 to a marked degree. Finally, the entire progress of development 

 is frequently stopped in nature by removing the supply of oxy- 

 gen or by sufficiently lowering the surrounding temperature, as 

 will be discussed in subsequent sections. 



3. CONTINUOUS AND DISCONTINUOUS MODES OF DEVELOPMENT 



Although, as stated in the foregoing section, each egg has a 

 more or less characteristic rate of development, this rate is not 

 uniform throughout the different developmental stages. All eggs 

 develop with rythmical changes in rate^ going alternately faster 

 and slower from stage to stage. Certain stages are passed very 

 ^'>| rapidly, almost suddenly, while others are slowly attained in a 

 tedious manner, yet the process of development is as a whole con- 

 tinuous. That is, development begins with fertilization which 

 is soon followed by cleavage, and then continues without inter- 

 ruption until a free living larva or young embryo is formed. 

 This then proceeds to grow and change until the adult structure 

 is attained. Such a continuous mode of development is most 

 common, indeed so common, that it is often carelessly consid- 

 ered to be universal, while a discontmuous mode, is looked upon 

 as something veiy strange or unusual and not as a phenomenon 

 extremely important in an understanding of the more common 

 continuous type of development. 



The continuous mode is found among the great majority of those 



animals in which the eggs develop in a uniform or homogeneous 



• environment, such as the sea-water. The general conditions of 



