74 EMBRYOLOGY IN ANTIQ^UITY [pt. ii 



refers to the heart as the first organ to be formed, and so as the seat 

 of the soul. But these minor sources contribute little to the progress 

 of the science, and it is upon the great work On the Generation of Animals 

 that Aristotle's well-deserved fame as an embryologist will always rest. 



If I have devoted a very large space to an account of Aristotle's 

 contributions to embryology, it is, firstly, because they are actually 

 greater in number than those of any other individual embryologist, 

 and secondly, because they had so profound an influence upon the 

 following twenty centuries. Embryology from the third century B.C. 

 to the seventeenth century a.d. is meaningless unless it is studied in 

 the light of Aristotle. 



His outstanding contributions to embryology may be put in the 

 following way: 



1. He carried to their logical conclusion the principles of the 

 observation of facts suggested by the unknown Hippocratic 

 embryologist, and added to them a discipline of classification 

 and correlation of facts which gave embryology a quite new 

 coherence. 



2. He introduced the comparative method into embryology, and 

 by studying a multitude of living forms was able to lay the 

 foundation for future science of the various ways in which 

 embryonic growth can take place. Thus he knew of oviparity, 

 ovoviviparity, and viviparity, and one of his distinctions is 

 substantially the same as that known to modern embryology 

 between holoblastic and meroblastic yolks. 



3 . He distinguished between primary and secondary sexual charac- 

 teristics. 



4. He pushed back the origin of sex-determination to the very 

 beginning of embryonic development. 



5. He associated regeneration phenomena with the embryonic 

 state. 



6. He reaUsed that the previous speculations on the formation of 

 the embryo could be absorbed into the definite antithesis of pre- 

 formation and epigenesis, and he decided that the latter alterna- 

 tive was the true one. 



7. He put forward a conception of the unfertilised egg as a com- 

 plicated machine, the wheels of which would move and 

 perform their appointed function in due course when once the 

 master-lever had been released. 



