382 GENERAL BIOLOGY 



the sequences of development in the hen's egg up to the time of hatching. 

 Harvey was a pupil of Fabricius, and built upon the work of his master. 

 These men opposed the preformationists, and called their theory epigene- 

 sis which simply means that the embryo arises by a gradual differen- 

 tiation of unformed material in the egg. 



Malpighi in 1672 sent two important papers on Embryology to the 

 Royal Society, but apparently the time was not yet ripe for his work 

 and it was neglected for nearly a century. He stood with the epigenetic 

 group. 



Bonnet (1720-1793) was one of the important men at this time who 

 threw the weight of his influence with Haller toward the preforma- 

 tionists. 



At present embryologists hold, as was stated above, that there really 

 is an organization of some kind in both egg and sperm, but that no 

 embryonic shape has yet been established. The definite shape comes 

 forth only by the gradual differentiation of the unformed (but not un- 

 organized) matter. We may therefore say "the whole future organism 

 is potentially and materially implicit in the fertilized egg cell," which 

 means that both sides were partially right. 



However, the greatest name in embryology is von Baer (1792-1876). 

 His work was done in the thirties of the last century. He is the father 

 of comparative embryology. It was he who first noted and described 

 cleavage, germ-layers, tissue and organ differentiation, and gave us the 

 well-known "recapitulation theory," now often called Haeckers "Law of 

 Biogenesis," on account of Haeckel's popularization of it. It will be 

 remembered that this theory holds that embryos pass through the adult 

 stages of the race to which they belong. 



The origin of life has always been an interesting speculative subject 

 for thinking men, and many and mysterious are the ways in which life 

 was supposed to spring forth spontaneously. Aristotle thought that mice 

 developed from the river's mud, while later writers suggested that old 

 rags and cheese combined in a dark cellar would produce the same result. 

 The history of this subject makes more than fascinating reading. 



Francesco Redi (1626-1698) was probably the first man to demon- 

 strate experimentally that life did not spring forth spontaneously as com- 

 monly supposed. He placed very thin cloth over a dish containing de- 

 caying meat and found that when flies were thus prevented from coming 

 in contact with the meat, no maggots formed, although maggots were 

 always supposed to arise spontaneously from decaying meat. But Redi 

 himself found parasites of various kinds within the bodies of other ani- 

 mals, and these he could not account for, so his experiment, while a 

 classic, did not settle the problem for others any more than it did for 

 himself. The settling of this vexed question was left for Louis Pasteur 

 (1822-1895), who first showed that decay was not the cause of micro- 

 organisms but the result of them. His experiments were made while 



