SHAPE OF THE NUCLEUS 605 
Nuclear polymorphism is often—though by no means 
always—associated with the presence of multiple nucleoli as 
shown in Pl. 23, figs. 3 and 5. In elongated nuclei the nucleoli 
usually lie parallel to the main axis of the nucleus as depicted 
in Pl. 24, figs. 2 and 6, and in Text-fig. 10. 
But it is in developing oocytes that the relation between 
nucleoli and nucleus is particularly deceptive. In the earlier 
stages of development the nucleoli come to lie at the periphery 
of the nucleus, and when invaginations subsequently appear 
in the nuclear membrane, they do so opposite the nucleoli. 
Pl. 23, fig. 2, and Text-fig. 4 illustrate this, and they suggest 
the possibility of nuclear incisions being formed under the 
influence of the nucleoli. On the other hand, it must be 
observed that in the case of some nuclei, the indentations in 
which are obviously due to certain of the mechanical causes 
already considered, the nucleoli are yet often in relation to 
the blind ends of the pouches in the nuclear membrane. In 
muscle, too, infolded portions of the latter often come into 
contact with the nucleoli, though here again nuclear incisions 
are primarily mechanical in origin. And finally, there are 
cells the nuclei of which contain nucleoli and yet have a nuclear 
membrane of regular contour, as shown in PI. 28, fig. 11. 
The main outcome of all this is that the relations so often 
seen between nucleoli and nuclear invaginations are usually 
secondary, and that the position of the nucleoli in such instances 
is rather an effect than a cause. 
11. Cent Division anp NucuEarR DIFFERENTIATION. 
It is not without significance that mitoses are extremely 
rare—if not altogether absent—in cells the nuclei of which 
contain well-developed canaliculi or incisions. Such, at any 
rate, is the case with the following tissues in adult mammals : 
Non-striated muscle. 
The various segments of the uriary tubule in the kidney. 
The epithelium lining the vesicles of the thyroid gland. 
Nerve-cells. 
Our observations suggest that while highly developed nuclear 
NO. 260 Ss 
