REPRODUCTION AND HEREDITY 283 



found in the maternal body the most suitable conditions for 

 their further development. This hypothesis reduced the ovum 

 to the level of mere food-material, at the expense of which the 

 spermatozoa live and grow. Supporters of Leeuwenhoek's 

 doctrine even went so far as to contend that they had under the 

 microscope actually seen a minute ' homunculus ' in the sper- 

 matozoon, and observed its development. Though this fantastic 

 contention soon met with the liveliest contradiction, the real 

 nature of the spermatozoon remained completely unknown until 

 the nineteenth century ; only with the establishment of the cell- 

 theory and with the proof that ovum and spermatozoon are 

 simple cells, and that fertilization consists in the union of the 

 male with the female cell, the veil began to lift from this 

 mysterious phenomenon. 



We have already become acquainted with the structure of 

 different animal- ova and seen how their cell-form becomes quite 

 distinct. Indeed, the eggs of sponges, and of some jelly-fishes 

 bear in their appearance a striking resemblance to simple 

 Amoebae, and have by their protoplasmic pseudopodia retained a 

 certain independence of motion (compare fig. 31, 3). In those 

 cases, however, in which the cell-character of the ova appears 

 hidden the differences lie either in a large accumulation of food- 

 material for the growing embryo, or in other secondary protective 

 arrangements demanded by the conditions of their later fate. 



But quite differently formed are the spermatozoa (fig. 70). 

 No one would take these thread-like motile formations to be 

 simple cells. However, we are here aided by evolution, which 

 shows how these thin threads are transformed, step by step, 

 from normal cells. We need not follow this process of develop- 

 ment in all its highly complicated phases, but it will suffice for our 

 purpose to remember that the head of the spermatozoon corre- 

 sponds to the cell-nucleus, thejmiddle part to the centrosome, and 

 the long, thin tail essentially to the protoplasm. 



By far the largest part of the cell-plasm is during the develop- 

 ment of the spermatozoon rejected as useless ballast. The sper- 

 mazoon acquires thereby greater motility and is at the same time 

 prevented from developing independently before impregnation. 

 This is one of the methods of Nature to enforce amphimixis. 



Why do ova not divide, though they are liberally supplied 



