156 EXTERNAL FACTORS III. 9 



independence of the parts in the growth and differentiation of 

 the organism ; while some pursue their normal course, others 

 deviate from it. It follows that when such deformations are due 

 to changes in the external conditions the parts are not equally 

 sensitive to the unusual influence to which they are exposed. 

 Thus in the Frog embryos which exhibit persistent yolk-plugs 

 and open brains when grown in solutions of various kinds, the 

 yolk and the medullary folds are alone susceptible to the action 

 of the poison, other parts are unaffected and continue their 

 development as though under normal circumstances : or, again, 

 a sea-urchin passes through the early stages of segmentation and 

 gastrulation unchanged when placed in a sea-water from which 

 magnesium has been removed, but the subsequent differentiation 

 of the gut and the formation of the skeleton are abnormal; 

 magnesium is necessary for these, though not for the earlier 

 processes. A means is thus afforded of watching the behaviour 

 of one or more parts independently of others, as, for example, of 

 the animal cells in the gastrulation of the Frog's egg when the 

 yolk-cells are injured, and the most valuable information con- 

 tributed, often quite unexpectedly, to our understanding of the 

 events of normal ontogeny. 



Quite apart from this such experiments have already con- 

 tributed, and will probably contribute still more in the future, 

 to the study of variation. Between conspicuous monstrosities 

 and those milder abnormalities which are termed ' sports ' or 

 ( mutations ' there is every intermediate gradation, just as there 

 is, on the other hand, no sharply defined limit between these 

 discontinuous and those far smaller continuous variations to 

 which the term has been often exclusively applied. The embryo 

 is particularly sensitive to a change in its environment and reacts 

 to such change by a variation in its form of greater or less degree. 

 And not only that ; as Vernon has shown, these changes can pro- 

 duce also an alteration in the variability of the species ; and so 

 provide greater opportunities for the operation of natural selection. 



At the same time teratology is not the main inquiry with 

 which the experimental embryologist is concerned. The pro- 

 blem that confronts him is to determine the part played by 

 each factor of the external environment in the processes of 



