740 LIFE : OUTLINES OF GENERAL BIOLOGY 



In many cases, however, the reproductive cells are not recognisable 

 as such until a relatively late stage in development, and after the 

 differentiation of the body has made considerable progress. Weis- 

 mann's suggestion for such cases was simply that the reproductive 

 cells which form at a later stage the gonads of the offspring, are 

 those cells in whose nuclei there persists an intact or complete 

 representation of the qualities of the fertilised ovum which developed 

 into the said offspring. In plants and in simple animals like zoo- 

 phytes, there is a widespread persistence of this embryonic type of 

 cell, in which the cytoplasmic differentiation has not gone too far 

 to prevent in appropriate conditions the development of the com- 

 plete set of hereditary factors carried by the nuclear chromosomes. 

 Hence the familiar facility of asexual multiplication in such types, 

 and the rearing of a complete new organism from a representative 

 fragment, such as a portion of a Begonia leaf or an eighth of a 

 Hydra polyp. 



This idea of the continuity of the germ-plasm is of great importance 

 in enabling us to understand the fundamental fact that a specific 

 organisation persists. The parent is rather the trustee of the germ- 

 plasm than the producer of the offspring, though the complementary 

 nurturing factor is, of course, emphasised in cases like the gestation 

 of mammals. 



The idea which Weismann made current coin had been previously 

 hinted at — by Owen, Haeckel, Rauber, Jaeger, Nussbaum, and 

 Brooks; and it was clearly stated in 1875 by Galton, who pointed 

 out that the child is, in a sense, as old as the parent; for when the 

 parent's body is developing from the fertilised ovum, a residue of 

 unaltered germinal material is kept apart to form the reproductive 

 cells, one of which may become the starting-point of a child. But 

 for practical purposes it was Weismann (1885) who made the idea 

 one of the foundation-stones of biology: "In development a part 

 of the germ-plasm contained in the parent egg-cell is not used up 

 in the construction of the body of the offspring, but is reserved 

 unchanged for the formation of the germ-cells of the following 

 generation." 



RECAPITULATION DOCTRINE.— Long before the evolution 

 idea was accepted by zoologists, the idea was mooted (e.g. by 

 Meckel in 1821) that the stages in individual development corre- 

 spond to grades of organisation in the animal kingdom. Von Baer 

 called attention to common features observable in vertebrate 

 embryos in early stages, but he indicated at the same time that 

 there was a remarkable specificity almost from the first. Louis 

 Agassiz, in his essay on Classification (London, 1859), expressed 

 his belief in a correspondence between stages in embryonic develop- 

 ment and the grades of differentiation recognised in the classifica- 



