GENERAL EMBRYOLOGY 



in these conditions are often so extreme that the phenomena of 

 heredity are scarcely apparent and usually the modified organ- 

 ism does not come to maturity. 



We may now attempt to summarize a conservative concep- 

 tion of the relation of the structure of the germ cells to the pro- 

 cesses of development and heredity. The zygote is an organ- 

 ism, morphologically and physiologically specific. It possesses 



h 



FIG. 149. Effects of chemical alteration of the surrounding medium, upon the 

 development of the sea-urchin. From Jenkinson. a. Without OH; ciliated 

 solid blastula of Sphcer echinus, b. KOH has been added, c. Normal blastula 

 of Sphcer echinus, d. Blastula in a K-free medium, e. Reared in K-free medium 

 and replaced in normal sea- water (Sphcerechinus) . f. Sphcerechinus larva from a 

 medium devoid of Mg. g. Echinus pluteus with three-parted gut, mouth, and 

 ccelomic sacs, but neither skeleton nor arms; reared without CaCOs or CaSO4. 

 h. Normal pluteus of Echinus. 



polarity, symmetry, various forms of differentiated substance, 

 even organs, composed of subsidiary elements and capable of 

 performing definite and highly varied and specialized functions. 

 This organism in its parts and as a whole does certain things, 

 makes certain reactions, in a word, develops. The quality of 

 the developmental reactions is determined primarily by the 



