10 



INVERTEBRATE MORPHOLOGY. 



gradually separating from one another (Fig. 2, A] until they 

 lie at opposite poles of the nucleus, usually taking up a posi- 

 tion ninety degrees distant from the point at which the origi- 

 nal centrosome lay. During this process the radiating fila- 

 ments which surround the centrosome become especially 

 distinct and may be divided into two portions, those which 

 come in contact with the nucleus and which from their 

 appearance in later stages are termed the spindle-fibres, and 



FIG. 2. DIAGRAM SHOWING THE PHENOMENA OF CELL-DIVISION. 



A, separation of the ceutrosomes ; cbromatin iu skein-stage. 



B, fully formed spindle ; cbroniatiu loops formed. 



C, longitudinal division of the cbroniatiu loops. 



I), separation of cbroniatiu loops and commencement of the division of the 

 cytoplasm. 



those which radiate outwards and are lost in the cytoplasmic 

 network and form the aster. In the meantime, however, im- 

 portant changes have been taking place Avithin the nucleus. 

 The chromatin substance, Avhich original!}- Avas scattered in 

 a reticulum, begins to arrange itself in a baud (Fig. 2, ^1) 

 which Avith many turns traverses the nuclear substance, the 

 nucleoli Avhich Avere present at the same time gradually van- 

 ishing. This stage of the process is termed the skein stage. 



