GENERATION OF ANIMALS 



the modern doctrine of " axial gradients " (see 

 741 b 28, n.). 



4'. He correctly understood the functions of the 

 placenta and the umbilical cord ;> and 



5. He prefigured (see 772 b 13 ff.) -with wonderful 

 insight the cell-streams or morphogenetic move- 

 ments which are fundamental in embryonic 

 development during the period when the germ- 

 layers are taking up their definitive positions." 

 His dynamic view of the origin both of normal 

 structures and of monstrous de\'iations can be 

 fully appreciated only in the hght of modern 

 knowledge of the great part played by move- 

 ment, migration of cells, etc., in early embryonic 

 development. 



On the contrary side we must range such mistakes 

 as these : 



1. The insect larva, which Aristotle regarded as the 

 earUer stage of an egg, " an egg laid too soon," 

 has in fact passed the embryonic stage. 



2. Observations of newly -castrated animals led him 

 to regard the testes as of secondary importance. 



With regard to his famous doctrine that the male Theory of 

 supplies the Form and the female the Matter of the w°™ *?*^ 



'^f Matter in 



embryo (see 729 a 11), some rms understanding may repro- 

 easily arise. And also, vriih regard to his insistence ''"'^^'°'*- 

 upon the importance of the Final Cause, we find that 

 modern scientific opinion, follo\\ing the lead of 

 Francis Bacon, who led the attack upon Formal and 

 Final Causes, often tends to consider Aristotle's talk 



" See J. Needham, Biochemistry and Morphogenesis {1942), 

 where also the most modern views on the origin of monsters 

 will be found. On this subject, C. Dareste's Production 

 artificielle des monstruosite's (1877) is still the classical work. 



