66 THE AMERICAN MONTHLY [March, 



toderm only. The layer begins at the summit of the manubrium, 

 there being no ingrowth of the ectoderm, as in some of the 

 Coelenterata, to form a throat. The endoderm and ectoderm are 

 separated by a non-cellular layer, called \.\\q supporting lamella^ 

 and there is no considerable space between them, nor any fold- 

 ings in the endoderm, but it everywhere lies in direct contact 

 with the ectoderm. The endoderm cells of the stomach cavity 

 (see fig 5) are large vacuolated cells with nucleus, and drawn out 

 into long flagella or pseudopodia, or both, and the food is taken 

 by engulphing, and can be seen in the outer end of the cell, or in 

 some sections has been even caught in the act of being engulphed. 



Zoologists of the present day regard Hydra as a community of 

 cells comparable with Amoeba or other protozoa, which, by being 

 banded together and dividing the work of the community among 

 various sets of cells, make the body of the size and powers of 

 Hydra a possibility. The parceling out of the vital functions has 

 been along two lines, the functions of alimentation, or working the 

 food into usable sha{>e chemically, being assumed by the endoderm, 

 and the functions of sensation and motion, so necessary to the dis- 

 covery and capture of food, as well as to the general care and well- 

 being of the cell-whole, being the work of the ectoderm. Such a 

 division is called a division of physiological labor, and it is under- 

 stood that the building up of higher and better animal bodies is 

 not possible except as this principle is more and more perfectly 

 applied in the construction of bodies. While in dividing labor 

 some of the functions of the primary unspecialized cell are set 

 aside and others are more highly specialized, some of the primary 

 functions are always retained, viz., the power of nutrition or of 

 controlling chemical laws and the power of cell reproduction by 

 division. This conception of the body of Hydra as a colony of 

 cells introduces a new conception of the nature of an animal's in- 

 dividuality or personality. If in Amoeba and the Protozoa we con- 

 sider each single cell a person, why not so here.^ Our idea of 

 personality, however, was formed before we knew of the cell doc- 

 trine, and we shall hence have to notice that in our idea of the 

 personality of Hydra we have to do with a different order from 

 that we see in Amoeba, it being, in fact, some of the united per- 

 sonalities of the individual cells. It is not, therefore, possible to 

 compare Hydra as a whole with a protozoan, but we must com- 

 pare its celfs with Protozoa to get satisfactory ground for biolog- 

 ical studies. 



The life-histor}' of Hydra bears out this conception of it as a 

 colony of cells. The gonads* are composed of cells which have 

 specialized the function of cell division, but in two very different 

 ways, viz : in the production of ova^ and in the production of 

 sponnatozoa. Hydra can reproduce itself by budding, a portion 

 of the ectoderm and endoderm growing out to form a miniature 

 Hydra, which gradually attains full size and reproduces in turn in 



*See Geddes, E. lirilt., xx, p. 407, Art Reproduction; also, Evolution of Se:>v. 



