Relation between Nucleolus and Chromosomes. 51 



Very evident is the fact that the nucleolus undergoes dissolution. The 

 first stage of this dissolution is a fragmentation into larger and smaller 

 masses (figs. 46, 47, 48, 53, 58). Less evident is the ultimate fate of the 

 nucleolar fragments. Even before the rupture of the nuclear wall the 

 initial stages of the nucleolar dissolution begin (figs. 43, 50). It is usually 

 consummated by the time that the first polar spindle has revolved into the 

 radial position, all traces of either the plastin or chromatin elements of the 

 nucleolus having been lost. Occasionally, however, a small chromatic 

 nucleolus, " metanucleus," persists for a considerably longer time, even until 

 the first polar body is formed (figs. 55, 60, 61, 62). It is ultimately also 

 resorbed by the cytoplasm of the egg and is never seen until the first seg- 

 mentation, when it is passed to one of the blastomeres, as described by 

 Wheeler (1895) for Mysostoma glabrum (here persisting to 8-cell stage). 

 The dissolution of the nuclear wall and the fragmentation of the nucleo- 

 lus are synchronous processes. Undoubtedly both these processes contribute 

 to the decided change that the nuclear reticnlnm now undergoes. Imme- 

 diately prior to maturation the nuclear reticulum was achromatic and wide- 

 meshed (figs. 32, 35, 38). By the time the nuclear wall has partially 

 disappeared the network becomes markedly close-meshed and chromatic (figs. 



51, 52, 53, 54, 55). The meshwork takes on a characteristic beaded structure. 

 Such nuclear residuum (always closer meshed and deeper staining in iron 

 hematoxylin than the surrounding cytoplasm) is clearly seen to persist until 

 the time the first polar body is fully formed (figs. 52, 63). It accom- 

 panies the polar spindle in its progress to the periphery of the egg. The 

 major mass lies about the central pole, closely surrounding it and forming a 

 mantle about the spindle to about the middle, and parts of it are seen even as 

 far peripheralward as the distal pole of the spindle (fig. 67). Conklin 

 (1905) in Cynthia and Ciona, and Lillie (1906) in Ch&topterus, have de- 

 scribed this " residual substance " of the nucleus in detail and have succeeded 

 in tracing it through the early ontogenetic stages following fertilization. I 

 was unable satisfactorily to trace the "residual substance " in the eggs of 

 Asterias forbesii beyond the stage when the second polar spindle was being 

 formed (figs. 70, 71). It appears that at this stage it becomes assimilated 

 with the cytoplasm, probably contributing thereto the chromatin that it re- 

 ceived from the nucleolus at the time of its dissolution and so playing the 

 role of a " formative stuff." 



The nucleolar fragmentation and dissolution may occur in several dif- 

 ferent ways. Usually the nucleolus breaks up into several larger masses and 

 from these the chromatin gradually escapes in the shape of granules (viscid 

 drops) leaving the several large masses of plastin ground-substance (figs. 50, 



52, 53, 54). Sometimes all the chromatin leaves the plastin nucleolus in a 

 mass (fig. 38) and subsequently breaks up in the nucleus, the plastin being 

 gradually resorbed by the protoplasm. Sometimes the plastin ground-sub- 



