150 EMBRYOLOGY. 



he endeavored to establish, the form of a double cup with 

 coelenteric cavity and a primitive mouth, but may be greatly 

 altered, as in the most of the Vertebrates, by the deposition of 

 yolk-material in the egg, so that the original fundamental form is 

 scarcely recognisable. Consequently he distinguished, according to- 

 the kind of modification, different forms of the gastrula, as bell- 

 shaped, cap-shaped, disc-shaped, and vesicular yastrulce. He made 

 the various forms arise by a process of invagination from a still 

 simpler fundamental form, the blastula, which is the final result of 

 the cleavage process.* 



HAECKEL published his excellent gastraea-theory in two articles in 

 iheJenaischeZeitsckrift: (1) " Die GastraBatheorie, die phylogenetische- 

 Classification des Thierreichs, und die Homologie der Keimblatter," 

 (2) " Nachtrage zur Gastrseatheorie." 



At the same time with HAECKEL, RAY LANKESTER in England was 

 led to a similar theory, which he had worked out in a paper full of 

 new ideas : " On the Primitive Cell-layers of the Embryo as the Basis 

 of Genealogical Classification of Animals." 



Both HAECKEL and LANKESTER failed to point out how the forma- 

 tion of the gastrula takes place in some of the divisions of Verte- 

 brates in Fishes, Reptiles, Birds, and Mammals. Essential service- 

 in the establishment and explanation of numerous questions of detail, 

 which remained unsettled in the gastrsea-theory, has been rendered 

 by BALFOUR, VAN BENEDEN, GERLACH, GOETTE, HOFFMANN, ROLLER,. 

 RAUBER, RIJCKERT, SELENKA, DUVAL, and others. 



Thus through HAECKEL'S gastrsea-theory the following points were 

 gradually cleared up : (1) The two primary germ-layers, which form' 

 the foundation for the development of both Invertebrates and 



* It should be here stated that even OKEN and C. ERNST v. BAER had 

 set forth, although in a very indefinite manner, the importance of the vesicular 

 form for the development of the animal body. OKEN was an opponent of the 

 germ-layer theory of WOLFF. In a criticism of PANDER'S investigations he 

 exclaimed with emphasis and a certain justice : " The facts cannot be so. The 

 body arises out of vesicles and never out of layers," and he added the very 

 pertinent remark : " It appears to me as if it had been entirely forgotten that 

 the yolk and the yolk-membrane, which is a vesicle, belong essentially to the, 

 body of the germ ; that tbe embryo does not swim upon it like a fish in the 

 water, nor lie upon it like a funnel on a cask." 



In a similar manner BAER remarks, but without further expounding the 

 relation to the germ-layers : " Since the germ is the undeveloped animal itself^, 

 one can affirm, not without reason, that the simple vesicular form is the 

 common fundamental form, out of which all animals are developed, not only 

 ideally, but historically." 



