THE VITAL PHENOMENA OB' THE CELL 205 



visible under all circumstances by means of certain definite stain- 

 ing 1 solutions. During division they are chiefly recognised by 

 means of their radiation figures, but these are not seen during rest. 

 Several data seem to point to the conclusion that the centro- 

 somes originate in the nucleus ; firstly, with a few exceptions, 

 nothing corresponding to a centrosome can be found in the proto- 

 plasm during rest ; secondly, at the commencement of division, 

 the centrosome is seen to be in immediate contact with the surface 

 of the nuclear membrane (Fig. 98), and only later on to move 

 further away from the nucleus into the 

 protoplasm ; thirdly, subsequent to this 

 appearance of the centrosome, the nuclear 

 membrane frequently collapses, just as if 

 nuclear sap had exuded through a small 

 aperture ; and fourthly, in many objects 

 the appearance of the centrosome is simul- FlG . 93. Nucleus of a 

 taneous with the disintegration of the nu- sperm-mother-cell of Ascans 



, ,. megaloi;efhala bivalens. The 



C ' e ^ 1- nuclein substance is ar- 



I have frequently occupied myself with ranged in threads which 



this question of the origin of the centro- ^C^J^ 



somes, and have expended in vain a great Appearance of the centro- 



deal of energy upon it. Latterly, during 8omee - Breaking up of the 



nj " , nucleolus. (Pi. III., Fig. 7.) 



my experiments upon the construction of 



the eggs and spermatozoa of Nematodes, I have again gone into 

 the subject, but have been unable to arrive at any definite con- 

 clusions. However, although at the present time the majority of 

 investigators consider that they belong to the protoplasm, yet a 

 certain amount of importance must be attached to the opposite 

 view, namely, that they have a nuclear origin. 



Finally, another point, which is as yet unexplained, is the fate 

 of the nncleoli, which disappear at the commencement of nuclear 

 division, and reappear in the daughter nuclei. What interchanges 

 of substances can have occurred in this process ? There are 

 exceptional difficulties in the way of the solution of this question, 

 since in many cases the nucleoli are composed of two chemically 

 different substances (vide p. 51). 



It appears probable to me that if we disregard the above- 

 mentioned connection with the centrosomes, the nucleoli, during 

 the preparation for division, become split up into small portions, 

 and become distributed upon the nuclear segments. 



In sperm-mother-cells of Ascaris, which have been hardened 



