HISTORY OF THE GERM-LAYER THEORY. 153 



beginning a paired structure, and compares it to the body -sacs 

 which are developed in Invertebrates by evagination from the 

 ccelenteron. BALFOOR justly alleges that the originally solid con- 

 dition of the two fundaments can have no weight against his inter- 

 pretation, since in numerous instances organs which ought properly 

 to contain cavities are developed solid, and subsequently become 

 hollow, as, for example, in many Echinoderms one encounters solid 

 cell-masses in place of hollow evaginations of the coelenteron. 



Led by theoretical considerations similar to those of the English 

 morphologists, my brother and I, by a thorough comparison of de- 

 velopmental and anatomical conditions, and with due regard to the 

 morphological and histological structure of organisms, then en- 

 deavored to bring to a solution this question of the day, the question 

 of the development of the body-cavity and the middle germ-layers, 

 by systematic investigations (published in " Studien zur Blatter- 

 theorie "), which extended over Invertebrates and Vertebrates. 

 The results of these series of investigations were published in two 

 articles : (1) in the " Ccelomtheorie, Versuch einer Erklarung des 

 mittleren Keimblattes," and (2) in the " Entwicklung des mittleren 

 Keimblattes der Wirbelthiere." 



In the first paper, in order to prepare the way, we were compelled to 

 give the term germ-layer a more precise definition. We designated 

 as such a layer of embryonic cells which are arranged like an 

 epithelium and serve for the limitation of the surface* of the body. 

 At the close of segmentation there is only one germ-layer present; 

 namely, the epithelium of the blastula. The remaining germ-layers 

 arise from it by the processes of invagination and evagination. The 

 inner germ-layer is formed by means of gastrulation, the two middle 

 germ-layers by the formation of the body-cavities, in that two body-sacs 

 are evaginated from the coelenteron, and grow out between and separate 

 the two primary germ-layers. There are, in the first place, animals 

 which are formed of two germ-layers, and possess in their bodies only 

 one cavity, a ccelenteron, produced by invagination (Ccelenterata 

 and Pseudocoelia), and, secondly, animals with four germ-layers, a 

 secondary intestine, and a body-cavity derived from the coslenteron 

 an enterocoal. To the two-layered animals belong the Coelenterates 

 and the Pseudocrels, but all four-layered animals are Enteroccels. 



From this standpoint we endeavored to prove that hitherto there 

 had been confused under the conception " middle germ-layer " two 

 things which are genetically, morphologically, and histologically 

 entirely different. 



