ELEMENTS OF STKUCTUEE. 



71 



Fig. 59. Two blood- 

 cells of the frog, 

 a 6, with granular 

 nuclei, as they pre- 

 sent themselves af- 

 ter treatment with 

 water. 



distinguished. This latter form is seen in the cells of involuntary muscle 

 just mentioned (fig. 56, Z>). In such cases the nucleoli 

 are frequently invisible. 



It often occurs also that elementary granules are laid 

 down in the nucleus, giving to it, when in large quanti- 

 ties, a rugged appearance, and precluding the possibility, 

 further, of the nucleolus being distinguished. It is thus 

 that the so-called granular nuclei have their origin. 

 Again, there are cells whose nuclei may be obscured by 

 an enveloping drop of oil. The former may be seen on 

 treating the blood-corpuscles of the lower vertebrates with 

 water (fig. 59), while the latter are of frequent occurrence among certain 

 cartilage cells. 



It is not always that we are able to make out the object in question in 

 the interior of animal cells : it is often hidden from view, as for instance, 

 in the living cell. We have already men- 

 tioned in a previous section that a rich de- 

 posit of elementary granules also, or pig- 

 ment molecules, may obscure the nucleus 

 (fig. 60). The same may be the case if the 

 cell-body be occupied by a quantity of fatty 

 matter; but very close scrutiny will always 

 reveal the nucleus to the observer after 

 a time. On the other hand, there are cells 

 in which such a covering up of the 

 nucleus cannot be thought of, in which 

 the contents appear perfectly clear, and 

 yet in which we can by no means in our 

 power render the nucleus visible. The 

 coloured blood-corpuscles of mature mammalia and human beings belong 

 to "this category (fig. 61); likewise the cells of the more superficial layers 

 of the epidermis which clothes the external sur- 

 face of the human body (fig. 62). But of both 

 these we know that they possesssed nuclei at an 

 earlier embryonic period. There are, consequently, 

 certain cells in our system whose nuclei disap- 

 pear usually at some period of their existence. 

 We may also remark here and there in tissues 

 whose cells are as a rule destined to retain their 

 nuclei for the whole life of the animal to whose 

 body they belong, an isolated cell without a nucleus among its complete 

 companions; but it must be looked upon as a rare anomaly. All such 

 non-nucleated cells are moreover incapable of 

 existing for any length of time, and are simply 

 on their way to dissolution as far as we know 

 at present. 



In contrast to the kind of cell just men- 

 tioned, we meet with others in which two or 

 even a greater number of nuclei exist. The 

 first case (fig. 63) is seen with comparative 

 frequency, and in very dissimilar tissues: 

 cells with many nuclei are rare, and found principally in the medulla 

 of bones, where they may contain ten, twenty, or even forty nuclei, and 



Fig. 60. Stellate cells filled with black 

 pigment. In two of the same we can 

 recognise the nucleus, but in the third 

 the latter is hidden by the quantity of 

 melanin granules contained in the cell- 

 body. 



Fig. 61. Coloured human 

 blood-corpuscles, a be. 



Fig. 62. 



-Epidermis cells without 

 nuclei. 



