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 

 cceleiiteron. BALFOUR 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 ccelenteron. 



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 surfaces of the body. 

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

 namely, the epithelium of the Uastula. 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 codenteron, 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 Pseudoccelia). and, secondly, animals with four germ-layers, a 

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

 an enteroccel. To the two-layered animals belong the Ccelenterates 

 and the Pseudoccels, but all four-layered animals are Enteroecels. 



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. 



