NOTES ON EMBRYOLOGY AND CLASSIFICATION. 405 



by the naked surface of all the constituent cells. At the 

 same time nothing could be more probable than that the outer 

 and the inner portions of each cell shoul.l acquire different 

 structure and properties, such as may, j t this day, be seen 

 to be acquired by the cells forming t.ie wall of the blastula- 

 phase of the jelly-fish, Geryonia (Figs. 5, 6). The liquid 

 within the cavity of the blastula was probably enough of a 

 special nature, and together with secreted products from the 

 cells, undigested food particles may have passed through the 

 substance of the cells into this blastocoei, and there have been 

 dissolved, so that an incipient digestive function was acquired 

 by the blastocoei. 



3. The diploblastic PLANULA=DiBLASTULA(Salensky). — 

 The diflFerentiation set up between the inner and the outer 

 portions of the cells forming in a single layer the wall of 

 the blastula now (we infer) advanced so far that each cell 

 divided into two, an inner cell and an outer cell. Possibly, 

 not all the cells composing the wall of the blastula took 

 part in this process. The result was the formation of an endo- 

 derm or enteric cell-layer by delamination. The cavity now 

 enclosed by its special layer of cells is conveniently termed 

 the ENTERON or ARCHENTERON (Urd^rm)j the cell-layer is 

 accordingly the enteric cell-layer ; on the other hand, the 

 outer set of cells usually known according to the termino- 

 logy introduced by Professor Allman in treating of the 

 Hydroid polyps as ectoderm, may also conveniently be 

 termed the deron or deric cell-layer. The delaminate Pla- 

 nula or Diblastula (a term which I adopt from Professor 

 Salensky) continued to nourish itself by the inception of 

 solid food by the naked protoplasm of its ectodermic cells. 

 We must, however, suppose that as the differentiation of 

 the deric and enteric cell- layers advanced, the inception of 

 nutriment became limited to one spot on the deric surface, 

 and that at this spot solid particles of food were passed 

 through the soft protoplasm into the enteron .there, to be 

 digested. The development of cilia on the general surface, 

 and of locomotion, Avould account for this localisation. A 

 rupture of the sac at this point and the establishment of an 

 open way into the already actively secreting and absorbing 

 digestive cavity, would constitute the mouth. 



Whatever view we take as to the original mode of forma- 

 tion of the digestive cavity or enteron, the difficulty has to 

 be encountered of forming a conception of the steps by which 

 the two vastly different modes of digestion which we meet 

 with in the animal series could pass one into the other. The 

 physiology of alimentation in a Protozoon, such as an Amoeba 



