188 THE SCIENCE OF BIOLOGY 



PROBLEMS OF DEVELOPMENT 



Embryology: The development of a fertilized egg into 

 an adult animal is a marvelous phenomenon. During the 

 nineteenth century the general course of this development 

 was ascertained for all the major groups of the animal king- 

 dom. The cruder misconceptions, regarding the links by 

 which an animal is connected with its descendants and its 

 ascendants, had been corrected before the middle of the 

 century. The cellular basis of development was made clear 

 during the fifty years which followed. But despite the 

 wealth of facts discovered by the embryologists, the marvel 

 of the developmental process has increased the more. Even 

 in the light of recent experimentation, it must be confessed 

 that relatively little is known regarding causation in develop- 

 ment, aside from a knowledge of the visible changes by 

 which the egg becomes the adult. These changes we know 

 tolerably well. What we wish to know is why particular 

 developmental changes occur as they do, why this or that 

 structure arises at a particular time, and what is the relation- 

 ship between internal and external phenomena. Now that 

 the sequence of structural changes has been made known, the 

 embryological problem has become the problem of under- 

 lying causation. In the solution of such a problem, there 

 must be recourse to experimentation. 



Fertilization illustrates the historical development of a 

 biological problem and also the progress from nineteenth to 

 twentieth century zoology. The fact that the semen of the 

 male was in some manner necessary for conception in higher 

 animals was known to the ancients. Aristotle wrote with 

 remarkable acumen upon the reproduction of animals. 

 The fact that animals of certain sorts arose from eggs was 

 known wherever the eggs were sufficiently large to be recog- 

 nizable. The eggs of birds and reptiles, and later (Redi, 

 1668) the eggs of smaller forms like insects were recognized 

 as the initial stages of development, but this did not explain 



