THE SEGMENTATION OF THE OVUM. 89 



between them. The nucleus during this process remains per- 

 fectly passive. 



In the second stage the nucleus becomes less distinct and 

 somewhat elongated, and around it the protoplasmic layer of 

 the earlier stage is arranged in the form of a disc-shaped ring, 

 compared by Fol to Saturn's ring. The protoplasmic rays still 

 take their origin from the perinuclear protoplasm. This stage 

 has a considerable duration (20 minutes). 



In the third stage the protoplasm around the nucleus 

 becomes transported to the two nuclear poles, at each of which 

 it forms a clear mass surrounded by a star-shaped figure 

 formed by radial striae. The nucleus is hardly visible in the 

 fresh condition, but when brought into view by reagents is found 

 to contain many highly refractive particles, and to be still 

 enveloped in a membrane. 



In the fourth stage the nucleus when treated by reagents has 

 assumed the well-known spindle form. The striae of which it is 

 composed are continuous from one end of the spindle to the 

 other and are thickened at the centre. The central thickenings 

 constitute the so-called nuclear plate. The clear protoplasmic 

 masses and stars are present as before at the apices of the 

 nucleus, and the rays of the latter converge as if they would 

 meet at the centre of the clear masses, but stop short at their 

 periphery. There is no trace of a membrane round either the 

 nuclear spindle or the clear masses ; and in the centre of the 

 latter is a collection of granules. The striae of the polar stars 

 are very fine but distinct. 



Between the stage with a completely formed spindle and the 

 previous one the intermediate steps have not been made out for 

 Toxopneustes ; but for Heteropods Fol has been able to demon- 

 strate that the striae of the spindle and their central thickenings 

 are formed, as in the case of the spindle derived from the 

 germinal vesicle, from the metamorphosis of the nuclear reticulum. 

 They commence to be formed at the two poles, and are then (in 

 Heteropods) in immediate contiguity with the striae of the stars. 

 The striae gradually grow towards the centre of the nucleus and 

 there meet. 



In the fifth stage the central thickenings of the spindle 

 separate into two sets, which travel symmetrically outwards 



