194 COMPARATIVE ANATOMY 



may be seen to multiply by binary division, the plane of 

 division always passing through the long axis of the body. 

 Transverse division has never been observed in any species 

 of Euglena. The division of the body begins at the anterior, 

 and passes gradually to the posterior end, so that in the middle 

 of the process one sees two organisms joined together, and 

 it would be difficult to say, if one did not follow it to the 

 end, whether there were two individuals conjugating or one in- 

 dividual dividing. Division of the cell-body is accompanied 

 by division of the nucleus, which goes through a peculiar form 

 of mitosis. It has already been said that the nucleus lies 

 nearer the posterior end of the body, that it is of oval shape, 

 and contains in its centre a body generally described as a 

 nucleolus ; but, as its behaviour differs from that of an ordinary 

 nucleolus, it will be best to call it by a special name viz. the 

 nucleocentrosome. The nucleocentrosome itself stains in- 

 tensely with certain aniline dyes, but not with those which 

 ordinarily stain chromatin, and thus it can easily be differenti- 

 ated from the true chromosomes, which are rod-like structures 

 arranged radially around it, and are so small and so numerous 

 that they cannot be counted. A nuclear membrane is present. 

 Preparatory to division the nucleocentrosome elongates 

 and becomes dumbbell-shaped, and the chromosomes are 

 at first disposed with regard to it like the barbs of a feather 

 to its axis. Eventually, as the nucleocentrosome continues to 

 elongate, the chromosomes come to lie parallel with it, and 

 when it assumes the shape shown in fig. 41, C, they are collected 

 in a ring round the middle of the fine thread which connects 

 its now swollen ends. At this stage the chromosomes split 

 longitudinally, and eventually their halves separate and travel 

 to the opposite ends of the nucleocentrosome. The thread 

 connecting the two ends of the latter then breaks, the whole 

 nucleus divides, and the chromosomes take up a resting position 

 around the two new nucleocentrosomes. The whole process 

 of mitosis goes on inside the nuclear membrane, as is common 

 in Protozoa, and the nucleocentrosome appears to take the 

 place, or, at any rate, to fulfil the functions, of the centrosomes 

 in normal mitosis. Previous to division, the flagellum of 

 Euglena is doubled ; not, it would appear, as the result of the 

 splitting of the original flagellum, but by growth of a new one. 

 It is not known whether there is any limit to the power of 



