50 Papers from the Marine Biological Laboratory at Tortugas. 



into two. Even shortly before this division astral rays are seen to take form 

 round the single centrosphere. These are undoubtedly formed from the 

 cytoplasmic reticulum, as Wilson has shown in the sea-urchin's egg. The 

 rays abut the nuclear wall and indent it slightly. The archoplasmic mass 

 now begins to elongate. Presently it assumes a dumb-bell shape, a bridge 

 of delicate fibers showing between the two globes. Each of the globes now 

 contains a centrosome (centriole of Boveri) surrounded by a centrosphere 

 and astral rays. The two centrosomes separate more and more, moving 

 over the surface of the nucleus, their astral radiations elongate and press 

 upon the nuclear wall, indenting it deeper and deeper until it finally ruptures, 

 allowing thus an intermingling of cytoplasm and nuclear content. Rupture 

 seems to be due to pressure plus some solvent influence of the rays. It is 

 apparent that no part of the achromatic structure of the first polar spindle 

 came from within the nucleus, for the wall in some cases is still intact when 

 the two centrosomes and asters (from which the spindle is spun) are 

 already formed (fig. 50). While the centrospheres enlarge and the astral 

 rays lengthen, the latter also increase perceptibly in thickness. This is par- 

 ticularly true of the traction fibers (Zugfasern), to which the chromosomes 

 become attached and by which they are drawn into the central spindle 

 (figs. 51, 52, 61, 62). The spindle is spun between the two centrosomes 

 by the union of the astral rays very much in the manner described by 

 Child (1896) in Arenicola marina. The outermost rays and those that do 

 not unite or blend to form the fibers of the central spindle interdigitate with 

 each other in the median plane (figs. 52, 64). The astral rays merge at 

 their distal ends into the general cytoreticulum. When the first polar 

 spindle is fully formed the centrospheres have a reticular or alveolar struc- 

 ture, and occasionally two centrosomes are seen (fig. 66). 



NUCLEOLAR CHANGES. 



When maturation is imminent the nucleolus is usually in a position on that 

 side of the nucleus nearest the cell wall (fig. 37, 50). In this position many 

 of the astral rays of the first polar spindle are extended directly upon it 

 (figs. 53, 58, 59). The fact that the nucleolus as well as the mass of chro- 

 mosomes and the occasional problematical body are all concentrated into this 

 narrow space, combined with the further fact that the nucleolus begins to 

 fragment and the chromosomes to scatter among these fragments, while the 

 astral rays are extended among them indiscriminately, makes it difficult 

 ordinarily to trace the fate of these several structures. However, excep- 

 tionally favorable conditions permit of observations which leave no doubt 

 as to the correct interpretation of the more complicated processes. Par- 

 ticularly favorable for such study is the condition where the chromosome 

 mass and nucleolus at the beginning of maturation are widely separated 

 (figs. 32, 38, 43, 46, 47, 48, 49). 



