SOMATOGENESIS 251 



It is as if heredity was represented by a long under- 

 ground tunnel. We are in the light at either end 

 and have made out to a considerable degree the details 

 at the entrance and exit, but we are still largely in 

 darkness throughout the passage-way itself. 



The science of embryology has given us a series of 

 flash-light pictures of what goes on in the tunnel of 

 development but of necessity its contribution has been 

 largely morphological. Consequently the geneticist 

 still awaits some torch-bearer who will reveal how an 

 invisible gene within a chromosome can give form and 

 substance to a definite visible unit character in an or- 

 ganism. Probably genetics has contributed more to 

 embryology than embryology to genetics in the past but 

 it is quite likely that the account will be more than 

 balanced in the future. 



The way in which germ-cells come by their potent 

 hereditary components, rather than how they make use 

 of them, has been the first and most natural problem 

 to engage the attention. The solution which satisfies 

 most biologists, who have considered the evidence, has 

 been found in the idea of the contmmty of the germ- 

 plasm, that is, that hereditary genes are not the product 

 and result of the body carrying them but are lineal 

 descendants of ancestral genes which have been housed 

 temporarily in other bodily domiciles in the past. 



The familiar miracle of how hereditary genes work 

 together to produce a new plant or animal is farther 

 from a satisfactory solution, yet there is no doubt that 

 some of the impending great discoveries in genetics are 

 sure to be exactly in this field. 



