202 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



as here used is that now generally applied to designate what was form- 

 erly implied by proto-vertebrce ; these embryonic segments represent the 

 rudiments of the paired muscular plates on the opposite sides of the 

 body of the adult.) The remarkable variation in the number of seg- 

 ments developed in embryos of different genera, as pointed out a little 

 way back, is also partly explained by the difference in the number of 

 muscular segments in the parent fishes of the different species. This 

 brings us to the recognition of the influence of that remarkable organic 

 force, heredity. 



XII. — SPECIFIC CHARACTER OF PROTOPLASM. 



Beginning with that very remarkable substance designated by the 

 name of protoplasm by Yon Mohl, sarcode by some writers, and the 

 physical basis of life by Huxley, from a nearly homogeneous state 

 in some protozoan types or in the yelks of some eggs, we pass in as- 

 cending steps from one type to. another until we find that out of it va- 

 rious tissues serving diverse uses have been differentiated. In the 

 most undifferentiated forms, in this remarkable substance there inheres a 

 power to feel and to distinguish objects which are fit for food from those 

 which are not fit. The near presence of food appears to determine 

 the lines along which the conscious living matter will travel, although 

 anything like visible sense organs are entirely wanting.* The rude 

 and primitive apparatus of movement before us in the amoeba foreshad- 

 ows what is possible with co-ordinated combinations of such elements 

 in more differentiated organisms. In the latter, similar minute lumps 

 of the living matter, the cellular elements, no longer retain in all parts 

 of the organism, of which they are at once the servants and members, 

 the power of feeling and moving, of being at one and the same time 

 nerve and muscle. Only in some degree do all the histological elements 

 of organisms retain this independent characteristic; it is probably this 

 hereditary legacy from the protozoan grade which constitutes what is 

 known to physiologists as the vis medicatrix naturce, nisus formativus, or 

 inherent remedial power. From the ccelenterata, some of which may be 

 cut in two and yet mutually reproduce the severed parts, up to the warm- 

 blooded mammalia where the powers of reparation are reduced to a min- 

 imum, such as the healing of wounds and the knitting together of broken 

 bones, and where a highly complicated and very fixed and special struct- 

 ure has put an end to the possibility of any power to multiply parts by 

 budding except in early embryonic states, but which rarely reach per- 

 fection of development except in the case of minor and unimportant 

 parts. This part of the subject has been more ably and more fully dis- 

 cussed in Darwin's Variation of Animals and Plants under Domestica- 

 tion, and is only iutroduced hero to illustrate the varying germinating 

 and reparative power of protoplasm in the various grades of the animal 

 kingdom. 



*See some remarkable cases described iu Leidy's Iihizopoih of North America. 



