400 PROFESSOR LANKESTER. 



the Diploblastica and Triploblastica, in the course of develop- 

 ment from the egg, after passing through a " polyplast'' con- 

 dition, enter upon the Planula pha^^e. The Planula was defined 

 as a sac, the wall of which is composed of two layers of cells, 

 an ectoderm and an endoderm. Such a Planula was presumed 

 to be the common ancestor of all Diploblastica and Tripo- 

 blastica, the former of which retained its essential structure 

 with small modifications, whilst the latter proceeded further 

 to add the third layer and the hsemolymph system connected 

 with it. The existence of an aperture leading into the cavity 

 of the two-cell-layered Planula was not an essential feature 

 of the ancestral form thus arrived at, for I was careful to 

 insist, in the essay referred to, that the two-cell- layered 

 Planula took its origin in the actual developmentof botli Diplo- 

 blastica and Triploblastica in two different ways, which I 

 designated respectively " delamination" and " invagination/' 

 When the deeper or endodermal layer of cells arose by 

 delamination from the inner face of a hollow polyplast, whose 

 wall was formed by one primitive layer of cells, a closed two- 

 cell-layered Planula was formed devoid of aperture, and sul)- 

 sequently a mouth was formed by a breaking through of the 

 Planula's wall and an ingrowth of ectodermal cells. This 

 mode of origin appeared to be confined to a few Zoophytes. 



The invaginate mode of origin — in which a pushing in of 

 the wall of a single-cell- layered sac gave rise to an internal 

 cell-layer — appeared to be by far the commoner mode of origin 

 of the two-cell-layered Planula, and in this case the cavity 

 formed by the invagination and bounded by the invaginated 

 endodermal cells is (for a time, at least) open to the exterior 

 by the orifice of invagination. I pointed out in the essav, 

 to which these remarks relate, that according to some 

 observers this " orifice of invagination" persists as the mouth 

 of the mature organism, whilst in other cases it closes up, 

 and again in other cases becomes the anus. The difficult 

 questions accordingly arose. Can the disruptive mouth of 

 ** delaminate Planulse" be identical or homogenous^ with the 

 mouth persisting from the primary orifice of invagination ? Is 

 the latter kind of mouih identical or homogenous with the 

 anal aperture of those organisms in which the orifice of 

 invagination persists as anus? Must we regard the orifice 

 of invagination as both mouth and anus, in fact as a " proc- 

 tostom" or " oranus." 



Professor Huxley, who in 1875 published some remarks 

 on this subject {" On the Classification of the Animal 



' Honiofjenous = derived from one and the same ancestral source. See 

 " Ou tlie use of the term Homology," ' Annals and Mag. Nat. Hist.,' 1870. 



