92 



SCIENCE 



[N. S. Vol. XXVII. No. 681 



albumin with forty carbon atoms may have 

 as many as one billion stereoisomers. 

 Reichert and Brown^ have recently found 

 that many genera and species of verte- 

 brates may be distingniished with certainty 

 by the properties of their hemoglobin 

 crystals. Here is positive proof that the 

 molecular constitution of at least one im- 

 portant substance differs in different 

 species, and if this be true of hgemogiobin 

 it may be safely assumed to be true of the 

 constituents of the germinal protoplasm. 



For the present, however, we must be 

 content to find the distinguishing char- 

 acteristics of different germ plasms in their 

 morphological and physiological properties 

 rather than in their molecular constitution. 

 And fortunately for the possibilities of re- 

 search the morphological and physiological 

 characteristics of the germ cells are suffi- 

 ciently numerous and evident to afford a 

 most fruitful and fascinating field for re- 

 search. 



In practically all theories of heredity it 

 is assumed that there is a specific ' ' in^ 

 heritance material," distinct from the gen-j 

 eral protoplasm, whose function is the 

 "transmission" of hereditary pi'operties 

 from generation to generation, and whose 

 characteristics, as compared with the gen- 

 eral protoplasm, are greater stability, inde- 

 pendence, and continuity. This is the Idio- 

 plasm of Nageli, the Germ-plasm of Weis- 

 mann. It is further assumed that this 

 germ plasm is itself composed of ultra- 

 microscopical unit's, which are capable of 

 undergoing transformation during the 

 course of development into the structures 

 of the adult. These are the hypothetical 

 Plastidules, Gemmules, Pangenes, Pla- 

 somes, Idioblasts, Ids, Determinants, Bio- 

 phores of various authors. However neces- 

 sary such units may be for a complete 



"By the kindness of the a,uthors I am permitted 

 to refer to this very important research which 

 will soon be published by the Carnegie Institution. 



philosophical explanation of development, 

 it must be confessed that at present they 

 constitute a purely hypothetical system 

 which may or may not correspond to 

 reality. We know that the germ cells are 

 exceedingly complex, that they contain 

 many visible units such as chromosomes, 

 chromomeres and microsomes, and that- 

 with every great improvement in the micro- 

 scope and in microscopical technique other 

 structures are made visible which were in- 

 visible before, and whether the hypothetical 

 units just named are present or not seems 

 to be a matter of no great importance, see- 

 ing that, so far as the analysis of the micro- 

 scope is able to go, there are differentiated 

 units which are combined into a system — 

 in short, there is organization. 



On the other hand the evidence in favor 

 of an inheritance material, which is dis- 

 tinct from the general protoplasm of the 

 germ and whose function is the reproduc- 

 tion of hereditary characters, is not con- 

 vincing. All the living substance of the 

 egg cell is converted by growth and dif- 

 ferentiation into the mature organism. 

 That there is a species plasm, or an indi- 

 vidual plasm, which is continuous from 

 generation to generation, and from which 

 all the qualities of the mature organism 

 are differentiated, is almost a certainty, but 

 there is no satisfactory evidence that this 

 substance is distinct from the general pro- 

 toplasm of the young germ cells. 



IV 



Differentiation, and hence heredity, con- 

 sists in the main in the appearance of un- 

 like substances in protoplasm and their 

 localization in definite regions or ceUa. 

 Such a definition is as applicable to the 

 latest stages of differentiation, such as the 

 formation of muscle fibers, as it is to the 

 earliest differentiations of the germ cells, 

 and the one is as truly a case of inheritance 

 as is the other. In short, different sub- 



