MUSEUM OF COMPARATIVE ZOÖLOGY. 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 (Wig. 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 1 0); 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- 
sally arranged in 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. Occasionally, however, the nuclei lie in 
