28 HISTORY OF THE EGG FROM FERTILIZATION TO CLEAVAGE. 



pronucleus, — becomes nucleopetal, — and is thenceforth called the "copu- 

 lation path" The angle formed by the first and second parts of the course 

 varies from U0° to 180°, according to the distance of the point of penetra- 

 tion from the pole. The first part of the course is accomplished through 

 a simple "movement of penetration"; the second, under the influence of 

 nuclear attraction. This is the only explanation now open to us for the 

 change of direction in the path of the spermatic bod)'. 



We have then to recognize two sorts of attraction, — one operating 

 between nuclei, and the other between nuclei and cytoplasm. We do 

 not attempt to say what the attraction is in either case, nor can we be 

 certain that the influences at work are properly described as those of 

 attraction. We insist only on the necessit}' of recognizing the facts estab- 

 lished by observation, and use " nuclear attraction " and " centripetal 

 attraction " only as provisional means of distinguishing phenomena for 

 which as yet we can find no physical explanation. 



t). TJic Hole of the Pronuclei. — That the male pronucleus pLiys a part 

 of leading importance in the fish egg was discovered in the fall of 1884, 

 and briefly announced in "The Pelagic Stages of Young Fishes," p. 19. But 

 we are indebted to Vejdovsky (No. 12), Boveri (No. 19), and Bohin (No. 14) 

 for the discovery that the male pronucleus may take the lead in the first 

 act of cleavage. Boveri's discovery that this is true even in a case where 

 the female pronucleus has the precedence in time of development, is 

 most interesting ; but we are not to conclude too hastily that this func- 

 tional importance belongs exclusively to the male pronucleus. Partheno- 

 genetic development with two polar globules (Platner), or even with one 

 (Weismann, Ischikawa, Bloehtnann), makes it certain that the primary 

 cleavage aster may arise independently of the spermatic element. If we 

 estimate aright the drift of recent discoveries in this line of work, the aster 

 is the important factor in developnent ; and if either pronucleus has any pre- 

 eminence over the other, it owes it entire/// to the accident of its association with 

 the aster. Considered apart from the aster, the pronuclei may still be re- 

 garded as morphological and physiological equivalents, although the facts 

 brought out by Vejdovsky and Boveri would appear, at first sight, to be 

 quite irreconcilable with such a view. 



If the aster cannot be considered an adjunct of one pronucleus more 

 than the other, it becomes more than questionable whether it is in any 

 case carried into the egg by the spermatozoon. Taking the evidence as 



