406 PROFESSOR LANKESTER. 



or an Infusorian, on the one hand, and of even the simplest 

 of the Enterozoa (the animals with an enteron or gut), differs 

 in the most important manner and to an extent which is 

 hardly sufficiently recognised. Whilst in the Protozoon the 

 raw, unprepared particle of solid food is plunged into the 

 living protoplasm of the cell, and, lying within a temporary 

 cavity partly filled with water, is digested within the proto- 

 plasm, ^no such inception of solid particles by the cells of 

 the enteron takes place, except perhaps in the sponges. 

 In the Enterozoa the food, though, as in all nonparasitic 

 animals, it is seized in the solid state, is yet not intro- 

 duced into the cell protoplasm in that state. It is dis- 

 solved in the enteron by the action of secretions there accu- 

 mulated, and passes only by diffusion into the protoplasm of 

 the enteric cells. The whole significance of the enteric 

 cavity — the physiological motive of its differentiation — ap- 

 pears to be that of a laboratory retort. The hypothesis of 

 its primitive ajDpearance as a closed cavity into which solid 

 food particles were passed through the protoplasm of the 

 cells as into a food-vacuole common to the cell colony, is in 

 harmony with this assumed physiological motive. On the 

 other hand, it does not seem possible to reconcile the phy- 

 siological significance of the enteron with the hypothesis that 

 it took its rise in a gradually deepening depression of the 

 surface of a spherical blastula — that is to say, by invagina- 

 tion. Such an area of depression is assumed by the invagina- 

 tion-hypothesis to have become the exclusively nutritive 

 area. Its cells must be actively taking in solid particles of 

 food at the surface like so many Amoebse. What motive is 

 there on such an assumption for the deepening of the de- 

 pression ? In what way can we suppose that the amoeboid 

 cells of this area came to cease the habit of seizing and 

 ingesting solid particles, and took to the outpouring of 

 digestive juices and the passive function of absorption ? By 

 what influences are we to suppose that the depression was 

 sufficiently deepened and its margin sufficiently narrowed to 

 retain a digestive fluid ? The answer to these questions 

 appears to me to involve more difficulty than we encounter 

 in tracing out the hypothesis of the origin of the enteric 

 cell-layer by delamination. This preliminary advantage of 

 the latter hypothesis is, wc shall see, independently strength- 

 ened and fortified by the facts and arguments with which we 

 meet at later stages. 



4. Formation of the stomod.eum and proctodeum. — The 

 breaking through of the mouth of the Diblastula, in the form 

 of a definite aperture, appears to have assumed neither 



