MODE OF ORIGIN OF CELLS. 33 



and are then again reformed, we have really not advanced one 

 step in the solution of the question, for even this rests on no 

 satisfactory evidence whatever. 



What we do know is, that in its earliest stage the fertilized egg 

 has no visible nucleus, and that the nucleus of the first segmen- 

 tation sphere originates in protoplasm; that when very young, 

 which must necessarily be its state in the segmentation sphere, 

 the nucleus consists of a little mass of substance, in which, 

 amongst other products of disintegration, albumen is found ; 

 lastly, that when old, and for this the unfertilized egg may be 

 taken as an example, it may become converted into a vesicle. 



Lionel Beale* has offered a plausible, though negative, ex- 

 planation of the significance of the nucleus. He applies the 

 term germinal matter " both to it and to protoplasm," and places 

 them in opposition to formed material which constitutes the 

 investing membrane. This view contains, at any rate, an 

 indication that the nucleus and protoplasm possess certain 

 characters in common. 



Our knowledge of the nature of nucleoli is still more imper- 

 fect than that which we possess respecting the nucleus. To 

 these also a special significance has been ascribed in the act of 

 propagation, Virchow having quite circumstantially described his 

 observation on the division of the nucleolus ; and this, indeed, 



is the whole extent of our knowledge. Ley dig refuses to 

 admit that they are of any importance; but we cannot go 

 so far as this, if we reflect that the nucleoli in many cases 

 develop into a vesicle, in which still smaller nucleoli may be 

 distinguished. 



CELL GENESIS. Schleiden advanced the theory that plants 

 originate exclusively in homologous constituents. He proved 

 that the nucleated cell was the only original component of the 



I embryo of the plant, and that the development of all tissues 

 might be referred generally to such cells. The formation of 

 these cells takes place in a plasma, the nuclei first appearing, 

 and then the investing membrane. The formative material, 

 however, is commonly found within previously existing cells. 

 * The Structure of Elementary Tissues, 1861. 



