58x THE HISTORY OF BIOLOGY 



to the influence of electricity, light, heat, chemical compounds; of these 

 the last in particular have produced results of great interest. Among such 

 may be quoted the experiments of Curt Herbst (born in 1866), Biitschli's 

 successor at Heidelberg. He placed sea-urchins' eggs in various saline solu- 

 tions; in lime-free sea-water the eggs disintegrate into their first cleavage 

 cells, w^hich give rise to dwarf larvas; the addition of lithium salt produces 

 quite abnormally developed larvas; treatment with sulphate-free sea-water 

 also causes peculiar malformations in various parts of the egg. Undoubtedly 

 the most remarkable of these chemical experiments in evolution are, how- 

 ever, those concerned with the initial development of the egg. In this field 

 Jacques Loeb not only has taken the initiative, but has also proved a leader. 

 Born in Germany of Jewish parents in 1859, he studied medicine in his na- 

 tive country, becoming a doctor and assistant lecturer in physiology at 

 Wiirzburg, but he soon migrated to America, where he held a number of 

 professorships, the last one being at the Rockefeller Institute in New York.^ 

 Already, in the eighties, R. Hertwig had observed that unfertilized sea- 

 urchins' eggs, if treated with a weak solution of strychnine, surround them- 

 selves with a membrane similar to that which appears after fertilization, 

 before the cleavage begins. Similar observations were made later by Morgan, 

 the well-known student of heredity. Loeb took up this problem for system- 

 atic revision and achieved results that at once attracted great attention and 

 in some directions produced very far-reaching conclusions and awakened 

 high expectations. After performing a series of experiments he worked out 

 a method of bringing the sea-urchin's eggs to the larval stage without ferti- 

 lization. This method is somewhat complicated; first of all, the eggs are 

 subjected to the influence of a weak organic acid, which induces the forma- , 

 tion of membrane, after which they are placed for a carefully fixed period 

 in sea-water, the salinity of which has been increased to one and one-half 

 times the normal and which besides has been mixed with soda, and finally 

 in normal sea-water, in which the development takes place. Several other 

 simpler methods have also produced results, but they have been few and 

 indefinite. The above method, however, subject to careful regulation, works 

 safely, although, contrary to Loeb's statements, the larv^ thus formed are 

 by no means invariably quite typical. On account of this, Loeb has tried to 

 ascertain the chemical compounds that induce development and has come 

 to the conclusion that the membrane formation is caused by the fat-dis- 

 solving capacity of the influencing acid, and that the spermatozoon, which 

 upon penetrating the egg has the same effect, must produce a similar sub- 

 stance. The "hypertonic" sea-water then used "corrects" this effect and 

 thereby contributes to the actual segmentation. Thus the multiplication of 



^ Loeb died in 1924. 



