AS THE ELEMENTAL GERM OF AN ORGANISM 337 



formation, namely, "that what cannot be perceived by the senses, 

 is not present in a preformed state in the germ ; that the germ at 

 the outset is nothing but unorganised matter, excreted from the 

 sexual organs of the parents, which in consequence of fertilisation, 

 gradually becomes organised during the process of development." 

 He states further that the organs differentiate themselves one after 

 another out of this unorganised germinal substance, and he tried 

 to actually demonstrate this process in individual cases. Thus he 

 showed how various plant organs gradually differentiate them- 

 selves out of the germinal substance, and in so doing undergo 

 alterations in their shape, and he pointed out that the intes- 

 tinal canal of a chick develops out of a leaf-shaped embryonic 

 structure. 



By thus basing his arguments upon accurate observation, in- 

 stead of upon preconceived notions, Wolff laid the foundation- 

 stone of the important hypothesis, which, based upon the theory 

 of development, has been gradually built up during the course of 

 this century. 



If we carefully compare these two theories, we see that neither 

 can be accepted in its entirety. Both have their weak points. 



The theory of Preformation is open to attack from the stand- 

 point of the evolutionists, since, in the higher organisms, each 

 individual is produced by the co-operation of two members of 

 separated sexes. When, later on, Leeuwenhoek discovered the 

 existence of spermatozoa as well as ova, an animated discussion 

 arose as to whether the egg or the spermatozoon constituted the 

 preformed germ. 



The hostile schools of the Ovists and Animalculists existed for 

 a century. The Ovists, such as, for instance, Spallanzani, stated 

 that the unfertilised ovum of a Frog was a diminutive Frog, 

 being of opinion that the spermatozoon only acted as a stimulating 

 agent, exciting vital activity and growth. The Animalculists, on 

 the other hand, by means of the magnifying glasses at their dis- 

 posal, discovered the presence of heads, arms, and legs in the 

 spermatozoon. They therefore considered that the egg was only 

 a suitable nutrient medium, which was necessary for the develop- 

 ment of the spermatozoon. 



Further, the theory of Preformation, more logically worked out, 

 leads to very serious difficulties. One such obstacle, which even 

 Haller and Spallanzani did not think could be overcome, was the 

 consideration that the germs of all the subsequent animals would 



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