THE PROCESS OF CLEAVAGE. 



53 



granules the nuclear plate of STRASBURGER. That which in the case 

 of the Sea-urchin ordinarily appears as a chromatic granule is found, 

 upon the employment of the highest magnifying powers, but 

 especially in the study of objects (fig. 28 A] more suitable for this 

 purpose, to be a small Y-shaped loop. The number of the loops or 

 chromosomes appears to be very definite, and subject to law for each 

 species of animal. 



At the tips of the spindle there may be demonstrated, in addi- 

 tion, two special and exceedingly minute bodies, one of which 

 occupies the exact centre of each of the two previously mentioned 

 systems of rays ; they are, in fact, to be regarded as the cause of the 



B 





r 



! 



Fig. 28. Diagram of nuclear division, after RABL. 



In figure A one sees the spindle, composed of delicate non-chromatic fibres, with the protoplasmic 

 radiations at its tips and the chromatic loops at its middle. The splitting of the filaments 

 of the latter has already taken place. In figure B the daughter-loops resulting from the 

 fission have moved apart in opposite directions. In figure C'they begin to arrange themselves 

 in a regular manner into two groups of loops. In figure D the. groups of daughter-loops lie 

 near the two poles of the spindle. 



latter. Inasmuch as during the elongation of the nucleus they are 

 to be found at each of its two poles, they may be especially designated 

 as polar corpuscles [or centrosomes]. During the whole process of the 

 division of nucleus and cell-body, it appears as though a directing 

 influence belongs to the two polar corpuscles. 



Important changes in the nuclear loops of the spindle take place 

 during later stages of the process of division. Each loop is split 

 lengthwise into two daughter-loops (fig. 28 A), as discovered by 

 FLEMMING and as confirmed since then by numerous other investi- 

 gators (STRASBURGER, HEUSER, VAN BENEDEN, RABL, and others). 

 These daughter-loops soon move apart toward the opposite ends of 

 the spindle (figs. 28 B, C ; see also the explanation of the figures), and 

 approach very closely to the polar corpuscles at their tips (fig. 28 D}, 



Thus by a complicated process a division of the stainable nuclear 

 substance into similar halves is brought about. As the immediate 



