MUSEUM OF COMPAKATIVE ZOOLOGY. 133 



that they have arisen from an elongated binucleate cell by the forma- 

 tion of a divisional cell wall. In one instance, I have found a cell wall 

 fully formed before division of the nucleus was completed (Fig. 27). It 

 cuts across the fine connecting thread at about the middle point of the 

 latter. This must be considered as in some degree abnormal, especially 

 since it was found in a serosa the nuclei of which had evidently degen- 

 erated. 



Although division of the cell is almost always accomplished by the 

 formation of a cell wall, I have found several constricted cells, showing 

 that division may be partly, or even wholly, effected in this manner. 

 Sometimes the constriction is so deep that the opposite walls meet 

 (Fig. 28) ; but it is more usual to find that, after the cell has become 

 considerably constricted, a cell wall is formed joining the inward curves 

 of the constriction, and completing the division. At first, I thought it 

 possible that the constriction was mechanically produced by the pres- 

 sure of growing cells on either side. But this would not explain the 

 invariable occurrence of the constriction at precisely the point where it 

 would take place in a free cell, — equidistant from the daughter nuclei. 

 Furthermore, the curvature of cell walls (see Fig. 13), which is almost 

 certainly caused by the growth of cells and consequent tension, has no 

 reference to the position of the nuclei. 



As far as can be judged, the daughter nuclei are, as a rule, of equal 

 size, and alike in shape. I have found many instances of beautifully 

 symmetrical division (Figs. 9 and 10) ; but the nuclei of the serosa are 

 not altogether exempt from the irregularities that seem to be inseparable 

 from amitotic division wherever it occurs. Sometimes the resulting 

 nuclei are obviously unequal (Fig. 13), even in young membranes; and 

 in old membranes, where the nuclei have undergone degeneration, not 

 only are the daughter nuclei extremely irregular in shape, but often 

 very dissimilar in size. 



Relations of the Nuclei to the Cell. — A very brief examination of a 

 preparation of the serosa convinces one that the nuclei are symmetri- 

 cally arranged iu the cells. When there is but one nucleus, it occupies 

 the centre of the cell ; when there are two or three nuclei, each presides 

 over a half or a third of the cytoplasm. This arrangement is so con- 

 stant, that any marked deviation from it catches the eye at once. In- 

 stances of decidedly unsymmetrical arrangement of nuclei, one of which 

 Figure 13 represents, are very unusual. As regards elongated cells, the 

 daughter nuclei lie in the long axis of the cell, and at approximately 

 equal distances from its ends. OccasioDally, however, the nuclei lie in 



