THE DEVELOPMENT OF ORGANISMS 739 



Organic Continuity. — ^The largest fact in heredity is the 

 tendency of like to beget like. Not only is there a general persistence 

 of specific characteristics from generation to generation, but minute 

 features, idiosyncrasies, and some pathological conditions, inborn in 

 the parents, may recur in the offspring. 



The reason for this persistence is very clearly seen in some types 

 which show what Weismann called "germinal continuity". At an 

 early stage in the development of the embryo of certain types, the 

 future reproductive cells of the organism are distinguishable from 

 those that are forming the body. The somatic cells develop in mani- 

 fold diversity, various hereditary characters finding expression in 

 the cytoplasm of each type — nervous, muscular, skeletal, and so 

 forth. In short, division of labour is established, and the body- 

 forming cells lose their likeness to the fertilised ovum of which they 

 are all, of course, lineal descendants. On the other hand, the future 



"^^mXG^ Aoyfi^ A^PyT^ ^■^• 



PR /FO. '■■•&.C.-"" /f.0. /fo. /fO. 



Fig. 120. 



Diagram of the Continuity of the Germ-Cell Lineage. PR, a succession of 

 unicellular organisms. To the right a succession of three multicellular 

 organisms each arising from a fertilised ovum (FO) ; the lineage of the 

 germ-cells (GC) is indicated by the basal row; SC, the somatic or body- 

 cells of the successive individuals (B I, B II, B III). 



reproductive cells are not implicated in forming the "body", but, 

 remaining virtually unchanged, continue the hereditary tradition 

 intact, and are thus able to start an offspring which must resemble 

 the parent, since it is made out of the same kind of material and 

 develops imder similar conditions. 



An early isolation of imspecialised reproductive ceUs has been 

 observed in the development of numerous types, e.g. threadworms, 

 leeches, arrow- worms, Polyzoa, some crustaceans (e.g. Cyclops 

 and Moina), some insects (e.g. Chironomus, the harlequin fly), 

 Phalangidae among Arachnids, Micrometrus aggregatus among 

 Teleostean fishes; and with less distinctness in some others. A cell 

 that gives rise to all the future reproductive cells is demonstrable 

 in the gastrula stage of Cyclops and among the first sixteen blasto- 

 meres in the threadworm Ascaris. If the fertilised ovum has certain 

 prospective characters or hereditary initiatives, say, a, b, c, . . . 

 X, y, z, then the single cell just referred to will have them, and so 

 will the cells to which it gives origin, including those liberated to 

 form a new generation. This is why like begets like. 



