DEVELOPMENT OF CELLS. 535 



from that formed by the coalescence of the cells them- 

 selves, which is acted upon by nitric acid, whilst that 

 formed from the nucleus resists it. The nucleus may be 

 on the exterior of the cell, and sometimes imbedded in the 

 wall. As an example of the first, may be instanced the 

 nucleus of the pigment-cell of the eye, fig. 271. If, in- 

 stead of the fibre being flat, it is cylindrical, it is formed 

 by the nucleus. When the nucleus is outside, the fibres 

 of the cell and those of the nucleus unite, fibre by the side 

 of fibre. As an example, we have the bone of the tooth ; 

 in this the fibres of the cell and nu- 

 cleus alternate. Again, if the nucleus ji 

 be arranged externally, it unites 

 across either behind or in front, and 

 thus a spiral filament is formed : 

 this is seen in cellular tissue and 

 tendon. The last-described form 

 may alternate with that described 

 before it, and there are all interme- Fig. 271. Slack pigment ceils 

 diate shades of difference. Nuclei f^ the humane^. 

 sometimes disappear when they are very nearly developed, 

 as in the cornea of the eye. 



Action of Cells. The subsequent changes of these depend 

 in a great degree on endosmosis. The nature of the mem- 

 brane is a necessary condition, for it determines the way 

 in which the stream should pass; and we find in general 

 that the current is from the rarer to the denser fluid. If 

 we take common salt and fill a tube with it, and put the 

 latter in water, we find that the salt rises, from the water 

 having passed into the tube, and at the same time the 

 water outside is saltish to the taste. It is not a constant 

 circumstance that the stream is from the rarer to the 

 denser fluid ; with alcohol and water, for instance, the 

 stream is from the latter to the former. Mineral substances 

 permit of endosmosis, as pipeclay and chalk, in a low 

 degree ; but sandstone does not allow of endosmosis at all ; 

 thus proving that there must be something in the nature 

 of the material to be permeated. 



As to the process of secretion, these depend in a great 

 degree on endosmosis, inasmuch as the materials are drawn 

 from the blood, and so thrown off. 



