66 TIIK AMERUWN MONTHLY [M.-vn-h, 



todcrm only. Tlic layer bc«jiiis at the suiuinitoftlic inaiuihriuin, 

 there heinjj no iiii^rovvth of the ectoderm, as in some of the 

 Ctvlenterata, to form a tliroat. The endoilerni ami ectoderm are 

 separateil l»y a non-cellular layer, called \.\\c suppor/inj^ /arnr//a ^ 

 ami there is no considerable space between them, nor any told- 

 injjs in the enJodorm, but it everywhere lies in ilirect contact 

 with the ectoderm. The endoderm cells of the stomach cavity 

 (see lit; 5 > are large vacuolated cells with nucleus, and drawn out 

 into Ion;; tlagella or |)seudo]>odia, or I)oth, and the food is taken 

 by enjjidphini;, and can be seen in tlie outer end of the cell, or in 

 some sections has been even caui^ht in the act of beinj; eny;idphed. 



Zo('iloj»ists of the present dav re«jard Ilvdra as a conmiunity of 

 cells comparat>le with Amu'ba or other proto/oa, 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 shape chemically, being assumed bv the endoderm, 

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

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

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

 division is called a di'ision 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 di\ iding labor 

 some of the functions of the primary unspecializcd 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 Amtcba 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 diflcreiit order from 

 that we see in Am<i>ba, 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 cells with Protozoa to get satisfactory ground for biolog- 

 ical studies. 



The life-history 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. i)ut in two very diflerent 

 ways, viz : in the production of ova. and in the pro(hiction of 

 spermatozoa. 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 Geddea, E Itrilt., xx, p. 407, Art Reproduction; also, Evolution of S^x. 



