282 LECTURES ON BIOLOGY 



hidden to the eye, it can be only too easily understood that 

 earlier investigators regarded the act of impregnation as the life- 

 creating element. Nevertheless, Aristotle was already on the 

 track of truth when he assumed that the mother supplies the 

 formative matter for the new creature, but that the father 

 causes the development. 



Owing to the difference in the size, animal ova were known 

 to investigators long before spermatozoa were discovered. The 

 scientists of the seventeenth and eighteenth centuries believed 

 almost universally that each ovum contained the developed 

 animal with all its different parts and organs, as it were, en 

 miniature, and that impregnation stimulated this animalcule 

 into growth and 'metamorphosis.' The celebrated Italian 

 physiologist and physicist Spallanzani thought, on the ground 

 of his investigations concerning the reproduction and develop- 

 ment of the frog, that as the frog developed by metamorphosis 

 from the tadpole, and this again directly from the ovum, the 

 egg itself must already contain a minute frog. Moreover, as the 

 fertilized egg is in appearance exactly like the unfertilized egg 

 which still adheres to the maternal ovary, he thought that the 

 embryos of frogs must be in existence in the maternal body 

 long before impregnation had taken place. In pursuing this 

 idea, therefore, investigators arrived at the conclusion that these 

 germs again contained the primary constituents (Anlagen) of 

 subsequent generations, so that according to them in the case of 

 man the ovary of Eve contained all the human beings that ever 

 lived and ever will be born. 



Difficulties arose for the supporters of this doctrine of ' pre- 

 formation,' when a disciple of the great Dutch physiologist, 

 Leeuwenhoek succeeded, in 1667, in discovering in the sperm 

 of a man suffering from gonorrhoea minute motile formations, 

 resembling microscopic tadpoles. At first these ' semen- worms' 

 were regarded as parasitic organisms, living in the seminal fluid 

 and being of no particular importance. But very soon Leeuwen- 

 hoek himself succeeded in demonstrating the presence of exactly 

 similar ' worms ' in the semen of many other animals, mammals, 

 amphibians, birds, fishes, molluscs, and so on. Finally, continued 

 investigations led him at last to the conviction that these sper- 

 matozoa were not the ova but the actual germs which merely 



