THE SEGMENTATION OF THE OVUM. 73 



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 stria?. 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 stria? 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 demonstrate 

 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 stria? of the stars. The stria? 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 towards the 

 clear masses, growing in size during the process. They remain 

 however united for a short time by delicate filaments named 

 by Fol connective filaments which very soon disappear. The clear 

 masses also increase in size. During this stage the protoplasm of the 

 ovum exhibits active amoeboid movements preparatory to division. 



In the sixth stage, which commences when the central thickenings 

 of the spindle have reached the clear polar masses, the division 

 of the ovum into two parts is effected by an equatorial constriction 

 at right angles to the long axis of the nucleus. The inner vitelline 

 membrane follows the furrow for a certain distance, but does not 

 divide with the ovum. All connection between the two parts of 

 the spindle becomes lost during this stage, and the thickenings 

 of the fibres of the spindle give rise to a number of spherical vesicular 

 bodies, which pass into the clear masses and become intermingled 

 with the granules which are placed there. The radii of the stars 

 now extend round the whole circumference of each of the clear masses. 



In the seventh stage the two clear masses become elongated 

 and travel towards the outer sides of their segments ; while 

 the radii connected with them become somewhat bent, as if a 

 certain amount of traction had been exercised on them in the 



