196 FERTILIZATION OF THE OVUM 



the spermatozoon, while the nuclei (chromatiii) are equally derived from 

 both germ-cells. 



There is very strong reason to accept the first part of this con- 

 clusion (applying to nucleus and cytoplasm), but the question of 

 the centrosomes remains an open one. The array of evidence given 

 above, derived from the study of so many diverse groups, seems to 

 place Boveri's lucid and enticing hypothesis upon a strong foundation. 

 Two essential points still remain, however, to be determined : first, 

 whether the facts observed in Ascaris, Echinoderms, Physa, Thalas- 

 sema, and the like, are typical of all forms of fertilization ; and, second, 

 whether, if so, the primary cleavage-centrosome is actually imported 

 into the egg by the spermatozoon or is only formed under its influence 

 out of the egg-substance. Roth these questions have been raised by 

 recent investigators, apparently on good evidence, and some of this 

 evidence is directly opposed to both of the principal assumptions of 

 Boveri's theory. Thus, Wheeler ('97) has found that in Myzostoma 

 both centrosomes are derived from the egg; Carnoy and Le Brun 

 ('97) maintain that in Ascaris one centrosome is derived from each of 

 the germ-nuclei; in some mollusks, according to MacFarland ('97) 

 and Lillie ('97), both egg-centrosomes and sperm-centrosomes dis- 

 appear, to be replaced by two centrosomes of unknown origin ; while 

 recent botanical workers are unable to find any centrosomes in fertili- 

 zation. These and other divergent results will be critically considered 

 beyond (p. 208) in connection with a more detailed examination of 

 the general subject. It may be pointed out here, however, that 

 recent researches on spermatogenesis (p. 170) render it nearly certain 

 that the centrosome of the sperm-aster cannot be the unmodified cen- 

 trosome of the spermatid, since the latter, in some cases, enlarges to 

 form a " middle-piece " or analogous structure that is far larger than 

 the sperm-centrosome. 



B. UNION OF THE GERM-CELLS 



It does not lie within the scope of this work to consider the 

 innumerable modes by which the germ-cells are brought together, 

 further than to recall the fact that their union may take place inside 

 the body of the mother or outside, and that in the latter case both 

 eggs and spermatozoa are as a rule discharged into the water, where 

 fertilization and development take place. The spermatozoa may 

 live for a long period, either before or after their discharge, without 

 losing their fertilizing power, and their movements may continue 

 throughout this period. In many cases they are motionless when 

 first discharged, and only begin their characteristic swimming move- 

 ments after coming in contact with the water. There is clear evi- 



