Januaey 14, 1916] 



SCIENCE 



47 



publication of Hertwig's first paper, sup- 

 plemented Hertwig's in several important 

 respects: (1) He observed the details of 

 penetration of the spermatozoon with a 

 clearness that has never been surpassed for 

 these forms. (2) He gave the first correct 

 account of the maturation divisions and 

 origin of the egg-nucleus (Hertwig re- 

 garded the latter as being the persistent nu- 

 cleus of the germinal vesicle) . (3) He paid 

 special attention to the origin of the fertili- 

 zation membrane and founded the classic 

 theory that it was an adaptation to prevent 

 polyspermy. (4) He was the first one ade- 

 quately to present the harmful efi:ects of 

 polyspermy. 



The period initiated by these two men 

 was characterized mainly by the repeated 

 demonstration of penetration of the sperma- 

 tozoon, the formation of a nucleus from the 

 sperm head, and the fusion of this nucleus 

 with the egg-nucleus. It was also grad- 

 ually demonstrated that the egg-nucleus is 

 genetically derived from the germinal ves- 

 icle by karyokinetic divisions. Thus the 

 genetic continuity of the germ nuclei with 

 nuclei of preceding cell generations was 

 established. As yet the character of the 

 fusion of egg and sperm nuclei had hardly 

 been raised, for the chromosome problems 

 and hypotheses were in a very nascent state. 

 Flemming's discoveries concerning chromo- 

 somes and their reproduction in karyoki- 

 nesis by splitting date only from 1876- 

 1878. 



All the problems of cell morphology were 

 in a fine state of fermentation during this 

 time, the really classic period of cell-mor- 

 phology; the foundations of our present 

 knowledge of cell-division were being laid; 

 before the decade 1870-1880 it had been 

 firmly established that cells arise only by 

 division from preexisting cells; but two 

 views of the origin of nuclei were still held, 

 one that of free formation, according to 



which the nuclei of daughter cells had no 

 genetic connection with the nucleus of the 

 mother cell, and the other that nuclei arise 

 by division from a preceding nucleus. 

 Little by little as a result of numerous in- 

 vestigations by many investigators, both 

 zoologists and botanists, the matter cleared 

 up. In 1878 Flemming was able to outline 

 the whole scheme of karyokinesis substan- 

 tially as we now understand it. 



The fundamental biological principle of 

 genetic continuity was foreshadowed by 

 the founders of the cell doctrine, and was 

 more or less distinctly foreseen by some of 

 their contemporaries, as in the case of Lal- 

 lemand. It was yet more clearly expressed 

 in Virchow's famous aphorism, omnis cell- 

 ula e cellula (1856) ; but it could not be- 

 come an established guiding principle in 

 genetic research until the entire cell-cycle 

 of the individual life history was worked 

 out in broad outline, until the process of 

 cell division was accurately ascertained 

 and applied to the genealogy of the germ- 

 cells, until the respective parts of ovum and 

 spermatozoon in the origin of the new gen- 

 eration were understood, nor until the hoary 

 doctrine of spontaneous generation was 

 banished bodily from the field of biology. 

 These were all accomplishments of that 

 great decade in biological research, 1870- 

 1880, for which the studies of the preceding 

 thirty years had furnished ample prepara- 

 tion. The entire superstructure of modern 

 genetic research rests upon the foundations 

 then laid. 



Professor Mark's paper on Limax (1881) 

 is a point of departure between the fertili- 

 zation studies of the seventies and those 

 that were to follow. Professor Mark ob- 

 served that the pronuclei come together, but 

 do not fuse to form a first cleavage nucleus, 

 as had been described for other animals. 



The first cleavage nucleus does not have a mor- 

 phological existence. 



