232 BIOLOGY AND ITS MAKERS 



zoologists has been in a measure embryological. Many of 

 them have produced beautiful and important researches, but 

 the work is too extended to admit of review in this connection. 



Oskar Hertwig, of Berlin (Fig. 70), is one of the repre- 

 sentative embryologists of Europe, while, in this country, 

 lights of the first magnitude are Brooks, Minot, Whitman, 

 E. B. Wilson, and others. 



Although no attempt is made to review the researches of 

 the vecent period, we cannot pass entirely without mention 

 the discovery of chromosomes, and of their reduction in the 

 ripening of the egg and in the formation of sperm. This has 

 thrown a flood of light on the phenomena of fertilization, and 

 has led to the recognition of chromosomes as probably the 

 bearers of heredity. The nature of fertilization, investigated 

 by Fol, O. Hertwig, and others, formed the starting-point for 

 a series of brilliant discoveries. 



The embryological investigations of the late Wilhelm His 

 (Fig. 71) are also deserving of especial notice. His luminous 

 researches on the development of the nervous system, the 

 origin of nerve fibers, and his analysis of the development of 

 the human embryo are all very important. 



Recent Tendencies. Experimental Embryology. — Soon 

 after the publication of Balfour's great work on " Comparative 

 Embryology," a new tendency in research began to appear 

 which led onward to the establishment of experimental em- 

 bryology. All previous work in this field had been concerned 

 with the structure, or architecture, of organisms, but now the 

 physiological side began to receive attention. Whitman has 

 stated with great aptness the interdependence of these two 

 lines of work, as follows: "Morphology raises the question, 

 How came the organic mechanism into existence? Has it 

 had a history, reaching its present stage of perfection through 

 a long series of gradations, the first term of which was a 

 relatively simple stage ? The embryological history is traced 



