10 



INVERTEBRATE MORPHOLOGY. 



gradually separating from one another (Fig. 2, A) until tliey 

 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-Jibres, and 



, a ''<-^ 



Pig. 2. — Diagram showing the Phenombna of Cell-division. 



A, separalion of the centrosomes ; cbromatin iu skein-stage. 



B. fully formed spiudle ; cbromatiu loops formed. 

 G, longitudiual division of the chromatin loops. 



D, separation of chromatin loops and commencement of the division of the 

 cytoplasm. 



those which radiate outwards and are lost in the cj'toplasmic 

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

 portant changes have been taking place within the nucleus. 

 The chromatin substance, which originally was scattered in 

 a reticulum, begins to arrange itself in a band (Fig. 2, A) 

 which with many turns traverses the nuclear substance, the 

 nucleoli which were present at the same time gradually van- 

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



