2 INTRODUCTION 



that the ability of reproducing the entire organism does not 

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

 special cells, ■which are 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 morphologically different repi-oductive 

 cells derived, as a rule, from two different individuals 

 {fertilization). This kind of reproduction, known as sexttal 

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

 however, non-sexual modes 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 compai'ative 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- 



