THEORY OF DEVELOPMENT 44* 



But if the various kinds of determinants are to get into ap- 

 propriate cell-groups, this cannot be a matter of chance. There- 

 fore, we must further postulate that from the first each deter- 

 minant has- a definite position in relation to its neighbours, 

 that the germ-plasm is not a mere loose aggregate of deter- 

 minants, but that it possesses a structure, an architecture, in 

 which the individual determinants have each their definite place. 

 It must be borne in mind that the germ-cell is a unity, a potential 

 organism, and not a heap of hereditary contributions. Weis- 

 mann supposes that the determinants are kept in relation to 

 one another by " vital affinities," by internal forces, some ex- 

 hibition of which is, indeed, demonstrable, as when a chromosome 

 or ribbon of ids splits into a double ribbon of ids. 



But if the mechanism of the distribution of determinants is 

 by cell- division one of the features of which is that the chro- 

 mosomes are halved with minutiose accuracy, so that each of the 

 two daughter-cells obtains a longitudinal half of each chromo- 

 some how does it come about that different determinants pass 

 into different cells of the embryo ? This difficulty led to the 

 further hypothesis that, while ids may divide into two identical 

 halves, they may also divide into two dissimilar halves. Weis- 

 mann supposed that besides integral (erbgleich) division of the 

 nucleus, there is also differential (erbungleich) division. The 

 reality of this differential division which many histologists 

 vigorously dispute cannot be directly demonstrated any more 

 than the splitting up of a complex molecule into different mole- 

 cules can be demonstrated. But in both cases we may infer 

 the occurrence from the results. It is not a hypothesis, but a 

 fact, that a cell may divide into two daughter- cells, one of which 

 goes to form ectoderm, while the other goes to form endoderm, 

 and this implies some sort of differential division. What in- 

 ternal forces or vital affinities are concerned we do not know. 



If an egg-cell can divide differentially into a primordial 

 ectoderm-cell and a primordial endoderm-cell, or into a formative 



