ONTOGENY 59 



before it has assumed the characteristic features of its 

 parents, then this type of embryo becomes known as a 

 larva. 



During the seventeenth and eighteenth centuries prac- 

 tically all embryology was dominated by the preforma- 

 tionist creed. According to this view, development was 

 simply the unfolding of a preformed miniature, which 

 only required nourishment for its growth to the adult 

 state. No part in the organism was formed before 

 another, all of them were created simultaneously. 

 Moreover, successive generations were explained by 

 supposing that the germ contained not only a pre- 

 formation of the organism into which it grew, but 

 numerous other preformed miniatures in ever-increasing 

 minuteness, so that at the creation the germs of over 

 two hundred thousand millions of men were created 

 and " packed away in the ovary of our mother Eve." 



Gradually, through the labours of Wolff, von Baer, and 

 numerous other workers, the foundations of modern 

 embryology were laid, and proof brought forward to 

 show that both the ovum and the spermatozoon were 

 cells, and were both necessary in fertilisation. Baffling 

 as the problem of fertilisation appeared, great advances 

 have been made towards a proper understanding of it, 

 and we know now that it is the orderly and intimate 

 union of a sperm-nucleus of paternal origin with an 

 ovum-nucleus of maternal origin, and the result thereof 

 a cell with a nucleus we may call the segmentation- 

 nucleus. At the same time a clear understanding of 

 the processes that lead up to the fertilisation stage in 

 the egg and sperm was obtained. 



