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it". This is not very revolutionary. But Leonardo was the first 

 embryologist to make any quantitative observations on embryonic 

 growth ; he defined, for instance, the length of a full-grown embryo 

 as one braccio and the adult as three times that. "The child", he 

 says, "grows daily far more when in the body of its mother than 

 when it is outside of the body and this teaches us why in the first 

 year when it finds itself outside the body of the mother, or, rather, 

 in the first 9 months, it does not double the size of the 9 months 

 when it found itself within the mother's body. Nor in 18 months 

 has it doubled the size it was 9 months after it was born, and thus 

 in every 9 months diminishing the quantity of such increase till 

 it has come to its greatest height." Here Leonardo touches on one 

 of the most modern quantitative aspects of embryology, and one 

 almost expects to see him exemplify his words with a graph until 

 one remembers with a shock that he lived two centuries before 

 Descartes and five before Minot. His numerical data may also have 

 included figures about the relative sizes of the parts, and the germ 

 of the line of research so successfully pursued by Scammon in our 

 own times may be found in the note "The liver is relatively much 

 larger in the foetus than in the grown man". Other quantitative 

 notes concern the length of the embryonic intestines as in the laconic 

 "20 braccia of bowels" and the statement that "the length of the 

 umbilical cord always equals the length of the foetal body in man 

 though not in animals".^ 



He said little about heredity, but in one place he mentions a case 

 of sexual intercourse between an Italian woman and an Ethiopian, 

 the outcome of which assured him that blackness was not due to the 

 direct action of the sun and that the "seed of the female was as potent 

 as that of the male in generation". Finally, the best instance of the 

 wideness of his thought appears in the note, "All seeds have an um- 

 bilical cord which breaks when the seed is mature. And similarly 

 they have matrix and secundines as the herbs and all the seeds 

 which grow in shells show". We have met this idea before in Hippo- 

 crates of Cos, and we shall find it again in Nathaniel Highmore. 



It is no coincidence that pictures of weights and cogs and pulleys 

 stand side by side in Leonardo's notes with anatomical drawings of 

 the embryo. As Hopstock says, "Leonardo arrives at the conclusion 

 that there is but one natural law which governs the world. Necessity. 



^ Leonardo would have enjoyed Fog's statistical study of 8000 umbUical cords (1930). 



