BIGELOW : NUCLEAR CYCLE OF GONIONEMUS MURBACHII. 355 



serves in its form an indication of its double origin (Fig. 147). In 

 this instance, then, there is no doubt that the membranes separating 

 the two nuclei actually break down, allowing their contents to flow 

 together and their networks to intermingle, so that this is not a case of 

 mere apposition of nuclei. 



Although this is the most common and therefore probably the typical 

 form of nuclear union, in a few instances I have observed modifications 

 of it which are due to the varying degree of differentiation attained by 

 the germ nuclei at the time of their union. Although the portion of 

 the membrane which at first separates them usually breaks down, it occa- 

 sionally persists (Fig. 148). In this example the chromatin of the egg 

 nucleus already has the usual form of a very loose threadwork, while 

 that of the sperm nucleus, now nearly as large as the egg nucleus, forms 

 a more compact network. 



Although the chromatin of the egg nucleus at the time of contact 

 is usually in the diffused condition, in one instance — a case of poly- 

 spermy (Fig. 142) — the chromatin net had reappeared by the time the 

 two nuclei had come in contact. In another case this stage had been 

 reached while the sperm nucleus still lay at some distance from the egg 

 nucleus. The sperm nucleus, too, shows considerable variation in its 

 nature at the time of meeting. 



In the majority of cases the first cleavage nucleus shows no trace of 

 its dual origin, the maternal and paternal -constituents being indistin- 

 guishable. But from the occasional existence of such cases as those just 

 described, in which the membrane separating the nuclei long persists 

 (Figs. 148, 149), it seems possible that actual fusion may sometimes 

 fail to take place. In any event, however, there is nothing in the history 

 of the fusion of the germ nuclei in Gonionemus to preclude an equal 

 distribution of both maternal and paternal chromosomes between the 

 two daughter nuclei of the first cleavage. 



Unfortunately, Gonionemus is not a satisfactory object on which to 

 trace the history of the sperm aster and its relation to the asters of the 

 first cleavage spindle, for it is impossible to detect the centrosome either 

 in the middle piece of the sperm after penetration or in the sperm aster ; 

 while in the cleavage asters themselves the demonstration of any such 

 structure is at best doubtful. 



E. The Cleavage Mitoses. 



In the study of the nuclear cycle of any animal the history of the 

 chromatin in the early cleavage mitoses is of great interest, both for the 



