THE STUDY OF ZOOLOGY 19 



The first step toward the final solution of this question was taken 

 by Carl von Nageli, who in 1844 stated, as a result of his studies, 

 that cells always come from preexisting cells and that they do 

 not arise from a generative matrix of some sort, as do crystals 

 from a liquid. Cells come from preexisting cells by a process of 

 cell division, but the discovery of the details of the process came 

 to light slowly because of their complicated nature. In cell 

 reproduction the division process is initiated in the nucleus, 

 where the transformation of the chromatin into chromosomes 

 takes place and where all of the intricacies of the division mecha- 

 nism seem to center. The importance of the nucleus in cell 

 division was recognized by Eduard Strasburger, a botanist, and 

 in 1879 led him to state that a nucleus always comes from a 

 preexisting nucleus, a conclusion confirmed in 1882 by W. Flem- 

 ming in studies of animal cells. It is largely due to the work of 

 these two men that the complicated details of cell division were 

 worked out. The term karyokinesis means division of the 

 nucleus and refers to the changes undergone by the nucleus in 

 the process of cell division. Mitosis is a synonym for karyo- 

 kinesis. A fuller discussion of mitosis is reserved for Chap. XI, 

 and it need only be mentioned at this point that, as a rule, 

 division of the nucleus is accompanied or followed by division 

 of the cytoplasm, resulting finally in the production of two cells 

 of equal size, each of which in the course of time grows to the 

 size of the parent cell. The division of the cytoplasm is called 

 cytokinesis. 



The development of an animal from an egg is usually preceded 

 by the fertilization of the egg by a spermatozoon. One of the 

 earliest observations of the entrance of the spermatozoon into the 

 egg was made in the case of the frog's egg by Newport in 1854, 

 but it was not until 1861 that it was clearly shown by the com- 

 parative anatomist Karl Gegenbaur that the egg is really a cell. 

 The same conclusion regarding the spermatozoon was reached 

 in 1865 by Schweigger-Seidel and La Vallette St. George. In 

 fertilization the essential feature is the union of the egg nucleus 

 with a nucleus derived from the spermatozoon, so that the 

 fertilized egg is really a cell with a nucleus of biparental origin. 

 These demonstrations of the cellular character of the egg and 

 spermatozoon have been verified many times since and have 

 tended to emphasize the importance of the cell in developmental 



