THE MORPHOLOGY OF FERTILIZATION 65 



are brought about by the sperm aster, the fibers of 

 which were supposed to act as contractile elements. 



The movements of the germ nuclei within the egg 

 depend on conditions of equilibrium of the various cell 

 constituents which constitute a definitely ordered stream 

 of events. The localization of the nucleus within the 

 cell is always determinate. We have therefore to con- 

 ceive that, as both sperm nucleus and egg nucleus are in 

 physiological relations to the same mass of cytoplasm, 

 which is preparing to divide, they must reach the same 

 position of equilibrium within the cell, and hence of 

 necessity meet. Their coming together is due, not 

 to mutual attraction, but to independent movements 

 toward the same part of the developing egg. This tend- 

 ency cannot, however, manifest itself until after matura- 

 tion is completed; hence the movements of the sperm 

 nucleus prior to the completion of maturation are not 

 always directed toward the ultimate place of union of 

 the germ nuclei, being under the influence of a different 

 condition of equilibrium of the egg cytoplasm. The 

 curved or bent path of the spermatozoon in certain cases 

 follows from this, and it is not found in the echinids, 

 where maturation is complete before fertilization. 



b) The two nuclei thus united may fuse together 

 to form a single nucleus called the first segmentation 

 nucleus, in which it may be difficult to distinguish the 

 two components for a certain period of time. But in 

 many, perhaps most, cases the changes preparatory to the 

 first cleavage of the egg begin before such a fusion occurs, 

 and in these cases it is easy to determine that each 

 germ nucleus contributes the same number of chromo- 

 somes to the first segmentation spindle (Fig. 90, b, d). 



