INDIVIDUALITY OF THE GERM-NUCLEI. 249 



ciation, but no actual union, and certainly no mingling of struc- 

 tural contents. Hence, although it is convenient to employ such 

 terms as "union of the germ-nuclei," and to speak of the "cleav- 

 age-nucleus" or "segmentation-nucleus" as if it represented a 

 single entity, it should be understood that such expressions are 

 loosely used, and the word "fusion" in this particular case is 

 altogether inapplicable. It is the purpose of this paper to show 

 that this unity or individuality of the respective germ-nuclei 

 is not confined to the fertilization stage, but persists with great 

 regularity during early cleavage, and with some apparent irregu- 

 larities throughout the later cleavage and even into the gastrula 

 stage. 



The length of time intervening between the meeting of the 

 germ-nuclei and the rupture of their nuclear membranes during 

 the formation of the first cleavage spindle must be considerable, 

 for resting germ-nuclei have been observed in eggs varying in 

 age from twelve to twenty-three hours after fertilization. The 

 precise time of formation of the first cleavage spindle has been 

 determined in only one egg, which was killed twenty-six hours 

 after artificial fertilization. 



Since the germ-nuclei usually come to lie in the same hori- 

 zontal plane, the most fruitful observations are obtained through 

 the study of horizontal sections. The two asters, formed by the 

 division of the single aster accompanying the sperm-nucleus 

 on its journey to meet the egg-nucleus, take up positions on 

 opposite sides of the resting germ-nuclei, in the same horizontal 

 plane, and close to the region of contact of the germ-nuclei 

 (Figs. 3 and 4). The initiation of the process of mitosis as 

 evidenced by the appearance of distinct chromosomes and the 

 rupture of the nuclear membranes does not take place in the 

 two germ-nuclei simultaneously; one of the germ-nuclei becomes 

 active somewhat in advance of the other (Figs. 5 and 6), thus 

 furnishing evidence of some degree of physiological as well as 

 structural independence. 



In the fully-formed mitotic figure the two groups of chromo- 

 somes, of maternal and paternal origin respectively, remain 

 visibly distinct (Fig. 7). This has been determined beyond a 



