INTRODUCTION ö 



represent the two primitive organs — or, as they are called, 

 the two primary germ-laijers — resulting from the differentia- 

 tion of the simple blastosphere. The gastrula-stage, which 

 recurs under various modifications in all the Metazoa, ap- 

 pears as the recapitulation of a hypothetical ancestral form 

 (Gastrea), which was characterized by the development of 

 the archenteron. Among the Metazoa now living many of 

 the Cnidaria have retained essentially the structure of this 

 hypothetical ancestor. In the more highly developed forms 

 the two primary germ-layers undergo various modifications, 

 whereby additional organs are differentiated. A third layer, 

 the tnesoderm or viiddle germ-layer, also grows in between 

 the two cell-layers. Concerning the origin of this we shall 

 speak farther on. Of the primary germ-layers, the entoderm 

 retains, even in the higher Metazoa, the function originally 

 belonging to it : that of receiving and digesting food. In 

 general it constitutes the epithelium of the mid-gut. From 

 the ectoderm, on the other hand, arise usually the epidermis, 

 and the nervous system, and sense organs, as well as the 

 epithelial lining of the stomodeal and proctodeal invagi- 

 nations. 



We have described above a method of origin of the 

 blastula- and gastrula-stages as it occurs in some, but by no 

 means all, Metazoa. It was chosen as the type because, 

 with due regard to the disturbing influences present, 

 many of the aberrant modes of development can readily be 

 reduced to the plan here presented. Frequently cleavage, 

 the formation of the blastosphere, and the process of gastru- 

 lation are modified by the presence and definite arrangement 

 of large masses of food-yolk. 



Certain eggs with little food-yolk approach most nearly to 

 the plan presented above {e.g., those of Amphioxus, Sagitta, 

 and the Echinoderms). In such cases cleavage results in the 

 production of blastomeres which are nearly uniform in size, 

 so that in the completed blastosphere only a slight difference 

 can be detected between the size of the blastomeres of the 

 animal and vegetative poles. However, even here those of 

 the vegetative pole are, as a rule, a little moi-e volujuinous. 

 This kind of cleavage is called total and equal cleavage. It 



