64 



PATHFINDERS OF PHYSIOLOGY 



ILLUSTRATIONS SHOW DIAGRAMATICALLY THE CELL AND 

 INDIRECT CELL DIVISION. 



Fig. 1. 



Fig. 2. 



Fig. 3. 



Fig. 4. 



Fig. 5. 



Fig. 6. 



Fig. 7. 



"The first change in the appearance of the nucleus which indicates that a 

 division is about to take place, consists in a rearrangement of the chromatin net 

 work, which now takes place on the appearance of a tangled thread (Fig. 2). 

 The outwardly directed loops of this skein often correspond to the seperate por- 

 tions into which the thread eventually breaks up. The thread gradually grows 

 shorter and thicker, and presently becomes divided into a number of pieces known 

 as chromosomes. In the chromosomes the shortening and thickening process is 

 continued until these bodies arrive finally at the form of stumpy rods, each of 

 which, often becomes bent into the form of a horse shoe. Meanwhile the nuclear 

 membrane, breaks down, so that the hyaline substance of the nucleus becomes 

 continuous with that of the cell body surrounding it. A fresh phenomenon now 

 becomes visible. A spindle-shaped arrangement makes it's appearance consisting 

 of a number of minute fibrils which connect together two points — the poles of 

 the spindle — situated at opposite ends of the cell. The chromosomes now change 

 their position so that they come to be in the plane of the equator of the spindle, 

 and about this line each chromosome splits longitudinally into two great por- 

 tions (Fig. 4 and 5). This splitting in the case of each chromosome takes place 

 in the equatorial plane of the spindle, so that one member of each pair of daugh- 

 ter chromosomes faces towards one pole of the spindle and the second towards 

 the other pole. The members of each pair of daughter chromosomes now begin 

 to move away from one towards the two poles of the spindle, and as they do so 

 the first indication of a dividing wall between the second new cells begins to 

 make its appearance in the equatorial plane. Arriving at the poles, the daughter 

 chromosomes begin to elongate and to put out processes which finally meet and 

 fuse with those of their neighbors to form the chromatin reticulum of the new 

 nuclei. (Fig. 7.) Surrounding each new nucleus, thus developing at either pole 

 of the now rapidly disappearing spindle, a new nuclear membrane makes it's ap- 

 pearance; the dividing wall in the position of the equator of the spindle develops 

 into a complete partition in tfie case of plants. (The animal cell is without a 

 cell wall.) The division into two new cells is thus completed. (Fig. 8.) Each 

 new cell is provided with a nucleus into which has entered precisely its fair 

 share of the chromatin which was present in the parent nucleus." 

 — Illustration and description after Locke. 



