30 KARYOKINESIS. 



processes which vary so much can have no fundamental or general significance. 1 

 And even if all these accounts are not accepted, they show that the problem is a 

 peculiarly difficult and complicated one and that it is still too early to formulate gen- 

 eralizations with regard to it. 



The evidence is certainly very convincing that in some cases both of the cleavage 

 centrosomes come from the sperm, but in other cases the evidence that they do not 

 have this origin amounts to a demonstration. This is shown in the clearest possible 

 manner in phenomena of normal and artificial parthenogenesis in which of necessity 

 the cleavage centrosomes must have their origin in the egg. Boveri recognizes par- 

 thenogenesis as an exception to his generalization and indicates that in such cases 

 the egg centrosome may not degenerate. This view presupposes a fundamental dif- 

 ference between amphigony and parthenogenesis such as does not actually exist. It 

 is well known that in certain animals the determining causes of amphigonic or par- 

 thenogenetic development are slight differences in extrinsic conditions; for example 

 there is no fundamental difference between the ova of the honey bee which develop 

 parthenogenetically and those which are fertilized, and how purely accidental in 

 this case are the causes which determine whether there shall be parthenogenesis 

 or amphigony. There is no world wide distinction between these two methods of 

 development and the differences as to the manner of origin of the cleavage centro- 

 somes cannot be fundamental. If in some species the egg centrosome is capable of 

 being preserved or reorganized, it is certainly quite possible that in others it may 

 not degenerate at all. There is therefore no a priore reason for supposing |that 

 Boveri's hypothesis is of general application and, as I have already attempted to 

 show, it is not in accord with all the facts. 



Certainly when one looks at the problem of fertilization from a general point 

 of view, when one considers the universal^ of sexual reproduction, when one 

 reflects upon the multitudes of exquisite adaptations which exist for securing the 

 union of egg and sperm he will be loath to believe that the essential feature of 

 fertilization is the addition of a centrosome to the egg cell or the supplying of a 

 stimulus to its development which is not needed in all cases and can as well be 

 supplied by changes in density, salinity, temperature, etc., as by the entrance of a 

 spermatozoon. 



III. Cleavage. 



I have already described the cell lineage or what may be called the external 

 phenomena of cleavage in Crepidula (Conklin '97) and must refer to that paper for 

 any detailed account of that process. I may be permitted here, however, to recall 

 a few of the more important features in the early cleavage. In this gasteropod, 

 as indeed also in all mollusks, the cleavage is of a peculiarly determinate, i. <?., con- 

 stant and differential, character. 



The first cleavage is equal and divides the egg into two blastomeres which are 

 approximately anterior and posterior in position ; the second cleavage is also equal, 

 dividing the egg into right and left portions, Plate V, figs. 80-88. Not only are the 



1 See foot-note. 



