MATURATION OF THE OVUM. 



to leave open for Asterias the question as to what parts of the germinal 

 vesicle are concerned in forming the first spindle. 



A clearer view of the phenomena which take place at this stage 

 has been obtained by Fol in the case of Heteropods (Pterotra- 

 chaea). In the ovum a few minutes after it has been laid the 

 germinal vesicle becomes very pale, and two stars make their 

 appearance round a clear substance near its poles. The nucleus 

 itself is somewhat elongated, and commences to exhibit at its 

 poles longitudinal striae, which gradually extend towards the 

 centre at the expense of the nuclear reticulum, from a metamor- 

 phosis of which they are directly derived. When the striae of the 

 two sides have nearly met, thickenings may be observed in the 

 recticulum between them, which give rise, where the striae of 

 the two sides unite, to the central thickenings of the fibres 

 (nuclear plate). In this way a complete nuclear spindle is 

 established 1 . 



The important result of Fol's observations on Heteropods, 

 which tallies also with what is found in Asterias, is that a spindle 

 with two stars at its poles is formed from the metamorphosis of 

 the germinal vesicle and surrounding protoplasm (fig. 25). 



Polar cells. The spindle has up to 

 this time been situated with its axis 

 parallel to the surface of the egg, but in 

 somewhat older specimens a vertical 

 spindle is found, with one end projecting 

 into a protoplasmic prominence which 

 makes its appearance on the surface of 

 the egg (fig. 26). Hertwig believes that 

 the spindle simply travels towards the 

 surface, and while doing so changes the 

 direction of its axis. Fol asserts, how- 

 ever, that this is not the case, but that 

 between the two phases of the spindle 

 an intermediate one is found in which a 

 spindle can no longer be seen in the egg, but its place is taken 

 by a body with a dentated outline. He has not been able to 

 arrive at a conclusion as to what meaning is to be attached to 



Fig. 26. PORTION OF 



OF THE DETACHMENT OF 

 THE FIRST POLAR BODY AND 

 THE WITHDRAWAL OF THE 

 REMAINING PART OF THE 

 SPINDLE WITHIN THE OVUM. 

 PICRIC ACID PREPARATION 



(copied from Fol). 



For the further details on the nuclear spindle vide the next Chapter. 



