PHYSIOLOGICAL 375 



network state, and there is a restoration of the apparently simple 

 status qito after a complex and meticulously precise process of 

 division. 



A study of fixed and stained material gives us the succession of 

 precise pictures which we have scantily outlined, but this is rather 

 disturbed by the results of fresh examination under the microscope 

 and with the micro-dissection apparatus. For the nucleus is seen to 

 be clear and structureless. It resembles a membranous bag filled 

 with fluid, though some kinds of nucleoli are more solid. Yet the 

 nuclear fluid is extraordinarily apt to set into a firm jelly, and this 

 is brought about by various stimuli, even including slight mechanical 

 injury. In the resting nucleus the jelly may show the chromatin 

 network structure; and, if it sets just before the division process 

 begins, the threads of the net may seem more delicate than usual. 

 During the process of division, when the membrane of the nucleus 

 has disappeared, the nuclear spindle has a strong tendency to set 

 or coagulate. One must remember that in the actively moving 

 Amoeba, giving off outflowing finger-like processes or pseudopodia, 

 there is an alternation of "sol" and "gel" states. The nucleus has a 

 strong tendency to pass into the "gel" state, when differentiations 

 previously invisible may become patent. But the results of studies 

 on living material all point to the conclusion that the living nucleus 

 at rest is a structureless fluid, except for certain kinds of nucleoli. 



What, then, is true of the chromosomes, believed to be the vehicles 

 of the hereditary characters ? In successfully fixed and stained cells, 

 such as the egg-cells of the horse's maw- worm, they stand out with 

 extraordinary clearness, substantial countable bodies, with precise 

 shapes (spherical, rod-like, V-like, K-like, and so on). They are 

 undoubtedly real and definite bodies, or perhaps one should say 

 differentiations. If the homely comparison is permissible, they may 

 be likened to as many minute and very unsubstantial sausages, 

 which readily change from "sol" to "gel", and are each surrounded 

 by a surface film. Besides the tj^ical mitotic cell-division we must 

 take account of other modes, notably amitotic and meiotic. 



Cell-inclusions. — Our discussion of the structure of protoplasm 

 expressly left out of count the various visible bodies included in the 

 clear fluid; these cannot, however, be entirely neglected. There are 

 usually granules in the cytoplasm, distinguishable as small micro- 

 somes and larger, somewhat unstable macrosomes. They refract 

 light sharply and are therefore easily visible. They are heavy and 

 can be separated from the protoplasm by means of the centrifuge: 

 they often show streaming movements which indicate the fluid state 

 of the protoplasm. The Echinoderm egg is a good example of a 

 cell crowded with granules. Certain specialised cells may contain 

 special inclusions ; for example, those of the green'plant and of a few 

 Protozoa contain plastids, in which chlorophyll is concentrated, and 



