152 EMBRYOLOGY. 



was created when KOWALEVSKY in 1871 published his " Embryology of 

 Sagitta," and showed how the cceleriteron of the gastrula was divided 

 by two folds into three cavities, into the secondary intestinal cavity 

 and into the body-cavities : this discovery was afterwards fully con- 

 firmed by the investigations of BtrrscHLi and the author. After a 

 short interval, KOWALEVSKY'S account of the development of Sagitta 

 was followed by his work on Brachiopods, in which he again enriched 

 science with the new and important fact, that in this class also the 

 body-cavity was formed in the same way as in the case of the 

 Chaetognaths. This was followed by his fundamental work on 

 Amphioxus. 



Through the important discoveries made on Invertebrates, HUXLEY, 

 LANKESTER, BALFOUR, my brother and I were stimulated to 

 theoretical speculations concerning the origin of the body-cavity 

 and the middle germ-layer in the animal kingdom. 



HUXLEY distinguished three kinds of body-cavity according to their 

 origin : (1) an enter ocoel, which arises as in Sagitta, etc., from evagi- 

 nations of the coelenteron ; (2) a schizoccel, which is developed by 

 means of fission in a mesodermal connective substance lying between 

 the integument and the intestine ; (3) an epiccel, which is formed by 

 an invagination of the surface of the body like the perithoracic 

 space of the Tunicates. The last kind, HUXLEY thinks, may perhaps 

 correspond to the pleuroperitoneal cavities of the Vertebrates. 



LANKESTER makes HUXLEY'S paper his starting-point. He gives 

 preference to the hypothesis of the common origin of the body- 

 cavity in all animals until decisive proof of diverse origins is 

 produced ; and, in fact, he makes the schizoccel arise out of the 

 enterocoel in the following manner. Evaginations of the coelenteron 

 have lost their lumen, and therefore are begun as solid cell-masses, 

 which only subsequently acquire a cavity. While LANKESTER in 

 this, as well as in a second publication, overlooks existing differences 

 in his effort to reduce everything to a single scheme, BALFOUR in 

 various essays takes more fully into account in his speculations the 

 actual condition of affairs ; he also limits himself chiefly to the 

 explanation of the conditions in Vertebrates. In investigating the 

 development of Selachians, he made the important discovery that 

 the middle germ-layer arises from the lateral margins of the primi- 

 tive mouth, and at first consists of two separate masses of cells, 

 which grow out forwards and laterally into the space between the 

 two primary germ-layers. Since in each cell-mass a separate cavity 

 soon makes its appearance, he designates the body-cavity as from the 



