PRODUCTION OF CELLS. xv 



cases the younger and more deeply seated cells have no envelope, but acquire 

 one before taking their place in the more superficial firm layers.* 



The envelope, when present, is thin, transparent, homogeneous, flexible, 

 and permeable to fluids. The contents differ greatly in different cells ; but 

 in those that form the first foundation of the tissues and organs in the embryo, 

 in young cells generally, and in some cells throughout their whole existence, 

 the contained matter is a peculiar semifluid substance, named %)rotoplasm, 

 and granular particles, mostly of a fatty nature, densely or sparingly mixed 

 with it. The protoplasm is transparent, colourless, not diffluent, but 

 tenacious and slimy, and under high magnifying powers is seen to contain 

 very fine molecules. In chemical properties it agrees generally with 

 albuminoid bodies, but in many animal cells it doubtless also includes other 

 organic principles, especially fat and glycogenous or amyloid matter. The 

 protoplasm is endowed with remarkable powers of contraction and motion, 

 to be afterwards referred to. But, while certain cells, as already said, 

 retain their primary constitution, others acquire very different matters : 

 many contain mucus ; the fat-cells are filled with oil ; the cells of glands 

 include the characteristic ingredients of the secretions ; the substance of the 

 red blood-cells is coloured, and certain cells are filled with particles of pig- 

 ment. Very commonly in vegetable cells the protoplasm occupies but a 

 small part of the space within, while the remainder is filled with watery 

 fluid with which the protoplasm does not mingle. 



Free Nuclei. Besides the bodies which have been called cells, corpuscles 

 having all the characters of cell-nuclei exist abundantly in various tissues, 

 such as the membranous walls of the capillaries, the sheaths of nerve-fibres, 

 muscular fibres, &c. But in some of these cases it is obvious, in others 

 highly probable, that the nuclei are associated with a certain amount of 

 protoplasm, although not in the usual mass and form of a cell. 



Production of Cells. Consistently with the present state of knowledge 

 on this subject, the following may be assigned as the several modes in which 

 cells and nuclei are observed to be produced in the animal system ; but 

 while the process varies in outward conditions in each of these cases, it can 

 scarcely be doubted that it will prove to be intrinsically and fundamentally 

 the same in all. 



a. In the Ovum. The ovum may be regarded as a cell derived from 

 the parent. In mammalia (fig. ix., A), it has a transparent but stout 

 external membrane (a) ; within this is the yelk (6), corresponding to the 

 cell-contents ; in the yelk is the germinal vesicle (c), including the germinal 

 spot (macula germinativa), which are comparable, respectively, to the 

 nucleus and nucleolus. The yelk consists originally of fine molecular 

 particles held together in a transparent tenacious matrix of protoplasm ; 

 and when once fertilisation has taken place, the yelk-mass undergoes a 

 process of subdivision or " segmentation," whereby it is fashioned into 



* The existence of animal cells destitute of envelope, although more insisted on of late 

 years, has been all along recognised in the study of cell-development, and was expressly 

 pointed out by Schwann himself (Microscopische Untersuchungen, &c.,p, 209). It has 

 appeared to some that another name should be used to designate bodies which thus exist 

 in a naked non- vesicular form. Briicke proposes to call them " elementary organisms," a 

 term too cumbrous for use ; as the first " shaped " products of organisation which appear 

 in the development of all but the lowest organised beings, they might be named " proto- 

 plasts," or, as that name has been already used in a widely different sense "monoplasts;" 

 but, after all, seeing the universal currency of the term " cell," it is probably most con- 

 venient and best to adhere to it, with the understanding that in many cases it is used in a 

 conventional sense. 



