208 BIOLOGY AND ITS MAKERS 
Swammerdam (1637-1680) supplied a somewhat better 
basis. He observed that the parts of the butterfly, and other 
insects as well, are discernible in the chrysalis stage. Also, 
on observing caterpillars just before going into the pupa 
condition, he saw in outline the organs of the future stage, 
and very naturally concluded that development consists of 
an expansion of already formed parts. 
A new feature was introduced through the discovery, hy 
Leeuwenhoek, about 1677,* of the fertilizing filaments of 
eggs. Soon after, controversies began to arise as to whether 
the embryo pre-existed in the sperm or in the egg. By 
Leeuwenhoek, Hartsoeker, and others the egg was looked 
upon as simply a vidus within which the sperm developed, 
and they asserted that the future animal existed in miniature 
in the sperm. ‘These controversies gave rise to the schools 
of the animalculists, who believed the sperm to be the animal 
germ, and of the ovulists, who contended for the ovum in that 
role. 
It is interesting to follow the metaphysical speculations 
which led to another aspect of the doctrine of pre-formation. 
There were those, notably Swammerdam, Leibnitz, and 
Bonnet, who did not hesitate to follow the idea to the logical 
consequence that, if the animal germ exists pre-formed, one 
gencration after another must be encased within it. This 
gave rise to the fanciful idea of encasement or emboiiement, 
which was so greatly elaborated by Bonnet and, by Leibnitz, 
applied to the development of the soul. Even Swammerdam 
(who, by the way, though a masterly observer, was always 
a poor generalizer) conceived of the germs of all forthcoming 
generations as having been located in the common mother 
Eve, all closely encased one within the other, like the boxes 
of a Japanese juggler. The end of the human race was con- 
* The discovery is also attributed to Harnm, a medical student, and to 
Hartsoeker, who claimed priority in the discovery. 
