DIFFERENTIATION, HEREDITY, SEX 



319 



ciliary movement depends upon the presence of magnesium, 

 and an excess of calcium results in the hypertrophy of the cilia; 

 sulphates are necessary also for the establishment of the funda- 

 mental structure of the embryo and for the formation of 

 pigment; magnesium, sulphates, and calcium carbonate are 



B 



-<m/ 



FIG. 148. Effects of magnesium chloride upon the development of the Teleost, 

 Fundulus. From Stockard. A. Normal fish, eight days after hatching. 

 M, mouth. B, C. Two views of fish, showing the fusion of the optic vesicles 

 as the result of treatment with MgCh. 



necessary for the development of a normal skeleton (Fig. 149). 



Certain optima exist for moisture, density, pressure, light and 

 temperature; in development as in later life, deviations from 

 the optimum condition, in either direction, affect the rate of 

 development rather than its character. The direction of 

 gravity takes an essential part in determining normal develop- 

 ment in a few cases, but ordinarily development is independent 

 of this factor. 



In general all of these conditions are involved not so much in 

 the regulation of development in specific and particular direc- 

 tions, as in determining whether it shall proceed at all or not. 

 Modifications of development produced by effective variations 



