DIFFERENTIATION OF ANIMALS. 501 



It is clear that the facts are capable of interpretation either upon the doc- 

 trine that external conditions guide or compel the cell in its development- 

 al career, or that it, by reason of an innate power, spontaneously pursues 

 a determinate course in spite of them ; determinate, because that power 

 is acting under a law. The mixed doctrine, which imputes the career of 

 development in part to the innate power of the cell, and in part to the in- 

 fluence of external conditions, it is needless for us here to consider. 



No doubt can be entertained of the fact that a cell or congeries of cells 

 will differentiate when submitted to new physical conditions while in the 

 act of development. Thus certain lichens pass into forms analogous to 

 algae if the normal conditions of their production be reversed if, instead 

 of developing in places that are dry and brightly illuminated, they are 

 supplied with moisture, and made to grow in obscurity; and, in like man- 

 ner, some of the fungi will simulate algse if they are compelled to vege- 

 tate in water. 



The separation of the organ for the reception of water and that for the 

 reception of carbon, which is first shadowed forth in the under and outer 

 surface of the lichens, is manifested in perfection by highly-developed 

 plants, in which the root discharges the former, and the leaves the latter 

 duty, and these are separated widely apart from each other by the as- 

 cending axis or stem. 



The remarks here made respecting plants might be repeated as re- 

 gards animals, which, during their development, exhibit the Differentiation 

 principle of differentiation even in a more striking way. of the animal 

 Thus, in the protozoa, as in the protophyta, cells undergo cel1 ' 

 duplication, and, by development in new positions, or under changed cir- 

 cumstances, exhibit differentiation. The trivial circumstances under 

 which new functions are assumed are well shown in Trembley's experi- 

 ments with the hydra* This polype, which is nothing more Experiments 

 than a gastric sac furnished with prehensile tentacles, re- with the hydra, 

 spires on its outer surface and digests on its inner; but so closely are 

 these functions blended together that, if the animal be turned inside out, 

 the surface that did respire will now digest, and that which did digest 

 will now respire. Indeed, we may in an ideal manner con- Ideal differen _ 

 ceive of the production of the more elementary animal forms tiation of ani- 

 as arising from a simple sac or bag, which, furnishing a start- 

 ing-point, exhibits its first acquirement of localization of function by the 

 doubling of one half into the other, thereby giving rise to a cup or pocket 

 shaped form, so that respiration and digestion, which were confusedly and 

 conjointly carried forw'ard upon the same surface, are now parted from 

 each other, the outside of the cup being devoted to the one, and the in- 

 side to the other. Increased endowments are obtained by crimping or 

 dividing the edge of the cup, prehensile organs of less or greater length 



