438 HEREDITY AND DEVELOPMENT 



contains both male and female determinants, analogous but 

 distinct. 



Summary of Weismann's Yiew. — " The germ-substance owes 

 its marvellous power of development not only to its chemico- 

 physical constitution, but to the fact that it consists of many 

 and different kinds of primary constituents — that is, of groups of 

 vital units equipped with the forces of life, and capable of inter- 

 posing actively and in a specific manner, but also capable of 

 remaining latent in a passive state until they are affected 

 by a liberating stimulus, and on this account able to interpose 

 successively in development. The germ- cell cannot be merely 

 a simple organism ; it must be a fabric made up of many 

 different organisms or units — a microcosm " (1904, vol. i. 

 p. 402). 



A living creature usually takes its origin from a fertilised egg- 

 cell, from a union of an ovum and a spermatozoon — two dimorphic 

 germ-cells. These germ-cells are descended by continuous cell- 

 division from the fertilised ova which gave rise to the two 

 parents ; they have retained the organisation of those fertilised 

 ova, and this organisation has its vehicle in the stainable 

 material of the nuclei — the germ-plasm. This germ-plasm con- 

 sists of several chromosomes or idants, each of which is made up 

 of several pieces or ids, each of which (here hypothesis begins), 

 is supposed to contain all the potentialities — generic, specific, 

 and individual — of a new organism. Each id is a microcosm 

 with an architecture which has been elaborated for ages ; it is 

 supposed to consist of numerous determinants, one for each part 

 of the organism that is capable of varying independently or of 

 being independently expressed during development. Lastly, 

 each determinant is pictured as consisting of a number of ulti- 

 mate vital particles or biophors, which are eventually liberated 

 in the cytoplasm of the various embryonic cells. All these units 

 of various grades are capable of growth and of multiplication 

 by division. 



