HISTORY OF THE GERM-LAYER THEORY. 149 



were equivalent to the serous and the mucous layers of BAER. 

 Soon after this (1853) ALLMAN introduced for the layers of the 

 Ccelenterates the names, which are now so much employed, ectoderm 

 and entoderm ; subsequently use was also made of these for designat- 

 ing the embryonic layers. 



The germ-layer theory was promoted to a still greater degree by 

 the Russian zoologist KOWALEVSKY, who made us acquainted in 

 numerous excellent detailed investigations with a profusion of 

 important facts concerning the embryology of Worms, Ccelenterates, 

 Molluscs, Brachiopods, Tunicates, and Arthropods. He produced 

 evidence that in all the Invertebrates which he investigated two 

 germ-layers are formed at the beginning of development, and that 

 in almost all cases, when the process of cleavage is at an end, a 

 cellular sac arises, and that this, by the infolding of a part of the 

 wall, becomes converted into a double cup, the cavity of which, 

 enclosed by two germ-layers, communicates with the outside by 

 means of an opening. He succeeded in establishing the existence 

 of this very important cup-shaped larva (gastrula) in many branches 

 of the animal kingdom. 



In this connection should be mentioned the services of several 

 other embryologists, who at a still earlier period had observed in 

 isolated cases the cup-shaped larva and its origin by means of 

 invagination. RUSCONI and REMAK had described the cup-shaped 

 larva of Amphibia, GEGENBAUR that of the Sagittse or arrow-worms, 

 MAX SCHULTZE that of Petromyzon. 



Whereas KOWALEVSKY by his series of investigations enriched our 

 knowledge of material facts, HAECKEL first sought to utilise the 

 same for a general theory, since by the process of morphological 

 comparison he brought into association hitherto disconnected obser- 

 vations. Starting from the development and the anatomy of the 

 Sponges, he compared the layer-like structure of the embryos of all 

 animals with the layer-like structure of the Ccelenterates, and pro- 

 duced as the fruit of this study the celebrated gastrcea-theory, which, 

 attacked on many sides at the time of its publication, has now 

 found in its essential substance general acceptance, and has given 

 the impetus to numerous investigations. HAECKEL showed that in 

 the development of the various classes of animals from the Sponges up 

 to Man a single form of the germ makes its appearance, the gastrula, 

 which consists of two cell- layers, and that the two cell-layers of 

 the various embryonic forms are comparable to one another or 

 homologous. The gastrula in its simplest condition presents, as 



