48 



SCIENCE 



[N. S. Vol. XLIII. No. lOUS 



The pronuclei persist after the appear- 

 ance of the cleavage centers, their mem- 

 branes then gradually disappear. 



In 1883 van Beneden published his now 

 classic paper on Ascaris: The pronuclei do 

 not unite, both are included in a single 

 amphiaster; each produces two chromo- 

 somes; these divide and their halves form 

 the daughter nuclei. In the nuclei of the 

 first two cells there are thus equal num- 

 bers of male and female elements, and there 

 are reasons to believe that even in these two 

 nuclei they do not fuse ; it is probable that 

 they remain distinct in all derivative cells, 

 including the immature eggs and sperma- 

 togonia. In the egg the chromatin is com- 

 posed of two distinct parts, and 



it is legitimate to suppose that each is the equiva- 

 lent of a male and a female chromosome, and that 

 in the formation of the polar globules each throws 

 out the male chromatin which it contains. 



Van Beneden by a veritable stroke of 

 genius thus anticipates the entire chromo- 

 some doctrine of the present time, even 

 though certain aspects of his interpretation 

 were not entirely fortunate : his conception 

 of the diploid cells as hermaphrodite, for 

 instance, and freeing of egg and sperm from 

 the male or female elements in maturation. 



With the establishment of the nuclear 

 theory, destined soon to be elevated into 

 the doctrine of chromosome individuality, 

 a certain duality of cell-life was recognized 

 in which nucleus and cytoplasm, however 

 interdependent, were regarded as playing 

 specific roles. But there was no logical 

 reason for stopping at duality, and the 

 centrosome soon came to be recognized 

 under van Beneden and Boveri's leadership 

 as a third organ of cell-life reproducing it- 

 self by division. The development of this 

 idea was due not only to studies of kary- 

 okinesis, but also to the series of fertiliza- 

 tion studies which began with Boveri's 

 classic papers on Ascaris (1887-1888). In 



these papers Boveri is convinced of the 

 necessity of making "the sharpest distinc- 

 tion between fertilization and heredity, i. e., 

 between the question how egg and sperma- 

 tozoon produce a cell capable of division, 

 and the question how these cells come to 

 be capable of reproducing the qualities of 

 the parents in the offspring"; this distinc- 

 tion has since been generally recognized. 

 Boveri's solution of the fertilization prob- 

 lem was in terms of the centrosome hypoth- 

 esis: the egg is devoid of the organ of 

 cell-division, the centrosome; capacity for 

 division, hence the initiation of the develop- 

 mental processes, is restored through the 

 introduction of a centrosome into the egg 

 by the spermatozoon. 



This conception exerted a dominating 

 influence on the sei'ies of fertilization stud- 

 ies which followed; the questions as to the 

 origin of the sperm aster with its contained 

 centrosome in the egg, and as to the genetic 

 continuity of the sperm centrosome with 

 the centrosomes of the cleavage amphiaster 

 were energetically investigated by a series 

 of students for the next fifteen years or 

 more, and similar studies have continued 

 with less energy down to the present time. 

 Collectively these publications constitute 

 a fairly exhaustive record of the morphol- 

 ogy of the fertilization process in animals, 

 a large part of which was furnished by 

 American students. 



The morphological analysis of fertiliza- 

 tion seems now to be fairly complete ; there 

 may still be disturbances such as recent at- 

 tempts to trace mitochondria back to the 

 sperm, which seems destined to share the 

 adverse fate of the similar attempt to trace 

 the centrosomes to the sperm; but there is 

 not likely to be any great modification of 

 the existing data, which seem to me to dem- 

 onstrate, effectively if not absolutely, that 

 the sperm head contains all the substances 

 necessary for fertilization. "We have thus 



