ü INTRODUCTION 



that the ability of i^eproducing the entire organism does not 

 belong alike to all the cells. It is confined rather to very 

 special cells, which ai^e known as reproductive cells (egg-cells 

 and sperm-cells) ; these are cells which for the most part 

 are developed only in definite regions of the organism {genital 

 organs^ gonads). The development of the Metazoan begins 

 with the fusion of twO' marphologically different reproducti*re 

 cells derived, as a rule, from two different individuals 

 (fertilization)) This kind of reproduction, known as sexual 

 reproduction,, is typical for all Metazoa. In many forms, 

 however^ non-sexual rtiodes of reproduction (by division or 

 budding) are interpolated in the life-history. If such an 

 interpolation is the rule^ so that two morphologically differ- 

 ent generations, one of which multiplies by sexual and the 

 other by non-sexual reproduction, regularly alternate with 

 each other, then this condition is known as metagenesis or 

 alternation of generations. It may also happen, however, that 

 there is a regular alternation of sexual generations, in one 

 of which reproduction is hermaphroditic or parthenogenetic, 

 while in the other it is- by means of separate sexes. Here 

 also there occurs a heteromorphous development of the two 

 generations. We call this condition heterogeny. 



Inasmuch as the individual Protozoan has the morpho- 

 logical value of a single cell, the embryology of the Protozoa 

 belongs to the province of cell morphology. For this reason 

 it is usually excluded from the domain of comparative em- 

 bryology of animals in the stricter sense ; in this book, too, 

 it will receive no consideration. Comparative embryology 

 accordingly has to do- with the development of the Metazoa, 

 and, above all, with their development from the fertilized 

 egg. Its chief problems consist in the investigation of the 

 formation of the germ-layers, the origin of organs, and the 

 development of the general form of the body. Its purpose 

 is the recognition of the laws of development, the determina- 

 tion of the homologies of organs, and the deduction of the 

 ancestral history of the Metazoa. 



The Metazoa constitute a single stem of the animal king- 

 dom. It is very «probable that all Metazoa can be referred 

 to a common ancestral form, and that certain correspond- 



