2 GENERAL EMBRYOLOGY 



or less precisely, the form and other characteristics of its 

 parents. The facts regarding all of these processes of develop- 

 ment, of the external and internal changes in form and structure 

 of the new organism, of the complex chain of processes leading 

 to its first formation, and of the role of external factors through- 

 out all of these, constitute the science of Embryology. We 

 may define Embryology briefly then, a^_Jhe__gcience__pl_the 

 genesis of the adult organism. 



As a general introduction to this subject we may first sketch 

 in outline the broad features of reproduction among the lower 

 animals, mentioning but a few of the almost infinitely varied 

 forms which this process assumes. It should be understood in 



FIG. 1. Simple fission in Amoeba vespertilio. After Doflein. A. Normal 

 vegetative form. B. Commencement of fission ("biscuit form"). C. Fission 

 nearly completed; separation of daughter cells, z, cells of an Alga, Zoochlorella. 



advance that the series of reproductive processes, of increasing 

 complexity, to be outlined, has little if any phyletic significance; 

 this arrangement is made for comparative purposes alone. 



The simplest and apparently the most primitive mode of re- 

 production is that known &s fission, characteristic of the single- 

 celled organisms, the Protozoa and Protophyta. In the case of 

 simple or Unary fission a separation of the nuclear material of 

 the cell into two separate masses is followed by a constriction 



