CHAPTER 16 

 EMBRYOLOGY 



The study of the development of the individual organism from the 

 zygote is the subject of the science of embryology. These complex 

 changes by which the single fertilized ovum develops into the many 

 different structures of the adult have long been a puzzle to the curious 

 observer. 



Aristotle (384-322 b.c.) made the first recorded observations of 

 the embryological development of the chick. The speculations which he 

 made marked the beginnings of a controversy which has only been re- 

 solved in recent times. He reasoned that the embryo must be either pre- 

 formed or that it developed from an amorphous beginning. From his 

 general observations, he concluded that the second explanation of the 

 two alternatives was probably the correct one. 



Leeuwenhoek, in the process of examining nearly everything, in 

 1677 made the discovery of spermatozoa in human semen. The large 

 eggs of various animals had, of course, already been observed for many 

 hundreds of years. Then, perhaps, due to the inadequacy of the micro- 

 scopes of the period, some individuals thought that miniature organisms 

 could be discerned within the sperm ; others thought that they must exist 

 within the egg. This led to the theory of prejormation by which it was 

 assumed that embryological development consisted only of the "unfold- 

 ing" of this miniature individual. Some thought that this miniature 

 was in the sperm and the egg only served for nutrition ; others assumed 

 that the sperm simply stimulated the unfolding of the miniature in the 

 egg. This led to some very remarkable conclusions, one of which 

 was that Eve must have had all preformed individuals of all succeeding 

 generations within her ovaries ! This taxed the imagination of some of 

 the eighteenth century scientists. 



When Wolfif in 1769 studied the development of the chick with 

 great care, he overthrew this idea of preformation. He saw that struc- 

 tures arose in the embryo by complete reorganization of previous struc- 



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