403 PROFESSOR LANKESTER. 



enunciated by Haeckel. Since the whole of recent theory 

 and inquiry as to the significance of the germ-layers of the 

 embryo in relation to the pedigree of the animal kingdom is, 

 in consequence of the interesting and vigorous writings of 

 Professor Haeckel, commonly spoken of (in this country at 

 least) as "Haeckel's Gastrsea theory ;'•' and since the distinc- 

 tive points of a similar but independent theory are liable in 

 consequence to be overlooked or misunderstood, I shall speak 

 of the latter as the Planula theory, and point out what from 

 the first has been the fundamental difference (coexisting with 

 a fundamental agreement) between Hseckel's Gastrsea theory 

 and the Planula theory. 



Objections to HaeckeVs views. — The developmental and 

 historic form which Haeckel's theory assumes, and to 

 which he gives the name Gastrula or Gastraea, is similar 

 to my diploblastie Planula, with this exception, that it 

 has a mouth. Haeckel definitely regards the orifice of 

 invagination or blastopore as the " Urmund/^ or primitive 

 mouth," and in his most recent writings has unreservedly 

 committed himself to the proposition that the ances- 

 tral Gastrsea originated by invagination, that the orifice 

 of invagination is the creature's mouth, and that those 

 recorded cases in which the embryonic two-cell-layered sac 

 is stated to arise by delamination are, like his own state- 

 ments relative to the delaminate origin of the endoderm in 

 calcareous sponges, cases in which observers have erro- 

 neously overlooked the invagination process. In my article 

 of May, 1873, I had in view the possibility of an identity 

 between blastopore and primitive mouth, and, indeed, sup- 

 posed that the Zoophytes might be distinguished from higher 

 organisms by the fact that they possessed the primitive, as 

 distinguished from a secondary mouth. On the other hand, 

 I have since seen reason to abandon altogether the notion 

 that the blastopore represents a mouth, and difter from Pro- 

 fessors Haeckel and Huxley on this point. 



I further disagree with them as to the universality of 

 the origin of the diploblastie phase by invagination. I 

 hold that we have at least one apparently Mell-observcd 

 case of the formation of an endoderm by delamination (' Fol. 

 Die erste Eutwickelung des Geryonideneies .Tenaische Zeit- 

 schr.,' vol. vii, p. 471), and, furtlier, tliat even without such 

 evidence (which ought to be re-examined) it is possible to 

 give a more satisfactory explanation of the early phenomena 

 of animal development on the hypothesis that the endoderm 

 originated primitii'ely by delamination, which has been super- 

 seded by invagination through the operation of readily con- 



