90 INTERNAL PHENOMENA. 



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 con- 

 nection 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 

 movement of the clear masses. Shortly afterwards the spherical 

 vesicles, each of which appears like a small nucleus and contains 

 a central nucleolus, begin to unite amongst themselves, and to 

 coalesce with the neighbouring granules. Those in each seg- 

 ment finally unite to form a nucleus which absorbs the substance 

 of the clear mass. The new nucleus is therefore partly derived 

 from the division of the old one and partly from the plasma of the 

 cell. The two segments formed by division are at first spherical, 

 but soon become flattened against each other. In each subse- 

 quent division of these cells the whole of the above changes are 

 repeated. 



The phenomena which have just been described would 

 appear to occur in the segmentation of ova with remarkable 

 constancy and without any very considerable variations. 



The division of the ovum constitutes a special case of cell division, and it 

 is important to determine to what extent the phenomena of ordinary cell 

 division are related to those which take place in the division of the ovum. 



