356 GENERAL EMBRYOLOGY 



layers of tissue, such as those of the chick, which seemed to give 

 rise to the chief organs of the embryo. These layers of sub- 

 stance were described and their significance recognized, by 

 such pioneers in embryology as C. F. Wolff (1768), Pander 

 (1817), and Von Baer (1828). The whole history of the forma- 

 tion of the systems and organs of the embryo could be read back 

 to these layers, but beyond these, few constant structural 

 features, susceptible of homolgy in different forms, could be 

 made out. The idea became very firmly fixed therefore, that 

 these layers were really the primary differentiations of the 

 embryo, and quite naturally their importance was strongly 

 emphasized. 



Subsequent to the statement and establishment of the cell 

 theory, the genesis of the germ layers was traced, the blastula 

 and gastrula fully described in a great variety of forms, and it 

 was found that in spite of the greatest diversity in the earlier 

 processes of development, the general character and structure 

 of the germ layers remained remarkably uniform. And not 

 only were the relations of the germ layers to one another quite 

 constant, but their relation to the tissues and organs of the 

 later embryo were subject to but little variation. In all forms 

 inner and outer layers (endoderm and ectoderm) were present, 

 and in all forms above the Coelenterates a definite intermediate 

 layer (mesoderm) was to be found. Moreover, in all of these 

 forms the outer layer gave rise to the whole nervous system, 

 central and peripheral, the essential parts of the sense organs, 

 the epidermis and its appendages; from the inner layer came the 

 lining of the digestive tract and its glandular appendages; 

 while the intermediate layer gave rise to the sustentative, 

 vascular, and muscular tissues throughout the body. All of 

 this was finally developed, notably by the Brothers Hertwig 

 (1879-1883), into a carefully and elaborately worked out Germ 

 Layer Theory, the essential points of which were that the three 

 germ layers are entirely homologous throughout the Metazoa, 

 excepting only the Porifera (the Coelenterates of course lacking 

 a middle layer), and that these layers truly represent the pri- 

 mary and fundamental homologies in the structure of the 



