264 ELEMENTS OF BIOLOGY 



Development. The development of an adult animal from an 

 Ggg constitutes one of the most interesting and at the same time 

 one of the most puzzling of the processes characteristic of living 

 material. An egg is alive and consequently carries on metabolism 

 and the transformation of energy into life processes. One may 

 measure the amount of oxygen taken in, the carbon dioxide given 

 off, the nitrogenous waste output, and other characteristics of the 

 egg and from the data obtained calculate with fair accuracy the 

 efficiency of the tgg in transforming energy, the materials being 

 used, and other physiological characteristics of the life of the egg. 

 Like physiological characters of later stages of development may be 

 determined with some assurance. Thus a series of determinations 

 may be made, each revealing information as to the nature of the 

 vital processes at any given stage. But the results give no clue what- 

 ever to the answer to the problem of why these stages succeed each 

 other nor as to how one stage is transformed into the next. Conse- 

 quently the facts of development as a physiological process are but 

 little known; it constitutes one of the most intriguing and at the 

 same time one of the most difficult fields of biological investigation. 

 Study and description of the changes in the form of the organism 

 from the tgg to the adult are much less difficult and have been 

 accomplished in a great many animals. Details of development vary 

 greatly; it is not the purpose of this discussion to enter into such 

 details nor does space permit. Certain basic similarities are found 

 throughout animal development, however, and attention will be 

 confined here to the events of development that are general, and to 

 useful comparisons. 



Following the union of the egg and sperm nuclei in the fer- 

 tilized egg, the events that follow may be divided into two main 

 categories: (a) Chemical changes in the transforming protoplasm, 

 which gradually convert the more or less generalized substance of 

 the egg toward the characteristically different types of protoplasm 

 found in the specialized tissues of the adult, are spoken of as dif- 



