150 EMBRYOLOGY. 



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

 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 gastrsea-theory in two articles in 

 iheJenaischeZeitschri/t: (1) " Die Gastrseatheorie, die phylogenetische 

 Classification des Thierreichs, und die Homologie der Keimblatter," 

 (2) " Nachtrage zur Gastraeatheorie." 



At the same time with HAECKEL, KAY 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 gastrcea-theory, has been rendered 

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

 RAUBER, RUCKERT, 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 OKEX 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 tlie 

 tody of the germ ; that the 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." 



