K. — BOTANY. 185 



liave, lor example, evideuce thai the iiiauifeotatiou ol a character may 

 be dependent upon the variation of internal conditions with age; in 

 other words, a time relation may be involved."* Or, again, upon the 

 state of general internal equihbrium resulting from the relation of one 

 morphological member or region to another. Thus removal of the 

 lamina of the leaf, so as to leave only the midrib, may cause the 

 mutilated individual to develop hairs on the stems and petioles in the 

 same envuionment in which the intact individual remains hairless. 

 Injury from attack by insects in a glabrous form may in like manner 

 lead to the production of hairs which, by their resemblance to those 

 of an allied species, show that the pathological condition set up has 

 caused genetic potentiality to become actual. But even if we exclude 

 the class of evidence to which objection on these grounds might be 

 made, there still remain various cases of normal types, where, unless 

 the behaviour of the chi'omosomes should point to a different explana- 

 tion, it seems most natural to assume that segregation takes place before 

 the reduction division. 



It has been argued from time to time that any scheme representing 

 the mechanism of Heredity which leaves out of account the cytoplasm 

 must prove inadequate. This general statement has been expressed in 

 more definite form by Loeb/" who holds that the egg cytoplasm 

 is to be looked upon as determining the broad outlines, in fact as 

 standing for the embryo 'in the rough,' upon which are impressed in 

 the course of development the characteristics controlled by the factors 

 segregated in the chromosomes. The arguments in favour- of the view 

 that the cytoplasm, apart from its general functions in connection with 

 growth and nutrition, is the seat of a particular hereditary process are 

 mainly derived from obsei-vation upon embryonic characters in certain 

 animals, chiefly Echinodemis, where the inheritance appears to be 

 purely maternal. It has been shown, however, that such female 

 prepotency is no indication that inheritance of the determining factors 

 takes place through the cytoplasm. Other causes may lead to this 

 result. It has been observed, for example, that hybrid sea-urchin lai-vae, 

 which at one season of the year were maternal in type, at another 

 were all paternal in character, showing that the result was due to some 

 effect of the environment. Again, where the hybrid plutei showed purely 

 maternal characters it was discovered by Baltzer -" that in the earliest 

 mitoses of the cross-fertilised eggs a certain number of chromosomes 

 fail to reach the poles, and are consequently left out of the daughter 

 nuclei. The chromosomes thus lost probably represent those contributed 

 by the male gamete, for in both parents certain individual chromosomes 

 can be identified owing to differences in shape and size. After this 

 process of elimination those characteristic of the male parent could 

 not be traced, whereas the one pair distinctive of the female parent 

 was still recognisable. In the reciprocal cross where the first mitosis 



" As in the case of characters which exhibit a regular change of phase. 

 f.g., the colour of white and cream Stocks is indistinguishable in the bud, and 

 a yellow-seeded Pea is gi-een at an earlier stage. 



" The Organism as a Whole. 1916. 



=" Archiv fur ZeUforschung. v., 1910. 



