INVAGINATION OR DELAMINATION 295 



origin of the two-layered sac which was very generally 

 admitted to be of constant occurrence in early embryogeny. 

 Ray Lankester, in his paper of 1873, and more fully in iS//, 1 

 propounded a " Planula " theory, according to which the 

 ancestral form of the Metazoa was a two-layered closed sac 

 formed typically by delamination, less often by invagination. 

 He denied that the invagination opening (which he 

 named the blastopore) represented the primitive mouth, 2 

 holding that this was typically formed by an "inruptive" 

 process at the anterior end of the planula. which led to the 

 formation of a " stomodaeum." A similar process at the 

 posterior end gave rise to the anus and the "proctodaeum." 



The question as to whether delamination or invagination 

 was to be considered the more primitive process was 

 discussed in detail by Balfour, 3 without, however, any very 

 definite conclusion being reached. He held that both 

 processes could be proved in certain cases to be purely 

 secondary or adaptive, and that accordingly there was 

 nothing to show that either of them reproduced the original 

 mode of transition from the Protozoa to the ancestral 

 two-layered Metazoa (p. 342). He by no means rejected 

 the theory that the Gastraea, "however evolved, was a 

 primitive form of the Metazoa," but, having regard to the 

 great variations shown in the relation of the blastopore to 

 mouth and anus (pp. 340-1), he was inclined to think that 

 if the gastrula had any ancestral characters at all, these 

 could only be of the most general kind. Balfour's attitude 

 perhaps best represents the general consensus of opinion with 

 regard to the Gastrsea theory. 



From the same origins as the Gastraea theory arose the 

 theory of the coelom. The term dates back to Haeckel in 

 1872, and the observations which first led up to the theory 

 were made by the men who supplied the foundations of the 

 Gastraea theory A. Agassiz, Metschnikoff and Kowalevsky. 



1 "Notes on the Embryology and Classification of the Animal 

 Kingdom," Q.J.M.S, (n.s.), xvii., pp. 399-454, 1877 



2 It was "part of the non-historic mechanism of growth" (loc. cif., p. 

 418). 



3 Treatise on Comparative Embryology, ii., chap, xiii., 1881. For 

 a modern 'discussion of this problem, see Hubrecht, Q./.Af.S., xlix., 

 1906. 



U 



