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CHAPTER XLI 



COMPARATIVE EMBRYOLOGY 



(By a. Richards, University of Oklahoma) 



Embryological knowledge has been a common property of mankind 

 since before the period of recorded history, and Aristotle embodied 

 with surprising accuracy in his extensive treatises on animal life a 

 great many facts in regard to the development of animals. Because 

 of the nature of embryos, however, it is obvious that there was a very 

 definite limit to any except the most superficial knowledge until the 

 development of the microscope early in the seventeenth century 

 opened the way for scientific embryology. The first extensive 

 study of the embryo was made by William Harvey and was pub- 

 lished in his work, Generation of Animals, in 1651. He was fol- 

 lowed by Malpighi, who published De formatione pulle in ova, in 

 1672. These investigators studied especially the chick embryo, but 

 Malpighi 's account includes also various invertebrates. In the mid- 

 dle of the eighteenth century came the great question of preforma- 

 tion versus epigenesis, with which the names of Wolff, Haller, and 

 Bonnet are connected. Workers later came to see that neither the 

 egg nor the sperm contains a perfectly formed small embryo, and 

 that development is a gradual growth from small beginnings. They 

 realized also that Wolff's claims that the germs develop from homo- 

 geneous material do not fully represent facts, although his concepts 

 were better than those of his opponents. To the student of present- 

 day embryology these difficulties are seen to be concerned with the 

 nature of organization of the embryo. The earlier investigators were 

 trying to understand the manner in which the mechanism of life 

 begins to operate, as well as the steps by which the structure of the 

 animal is produced. The next step came in 1828 when von Baer pub- 

 lished his germ-layer doctrine, according to which all the parts of an 

 embryo develop from three sheets of tissue, and the organisms are 

 formed by outpocketings, foldings, thickenings, and other mechanical 

 devices applied to these three sheets of tissue. This theory is said 

 to have established embryology as a science. It was a very 

 stimulating generalization and contains much of truth; yet it 



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