MATURATION OF THE OVUM 71 



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 

 \vith 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 Fig. 6. PORTION or 

 the spindle simply travels towards the 



surface, and while doing so changes the OF THE DETACHMENT 01 



.. r -rf i U THE FIRST POLAK BODY AND 



direction of its axis, rol asserts, how- THK W | T | IDRAW AL or THE 

 ever, that this is not the case, but that EEMAIMN.; i ART or THE 



SPINDLE WITHIN THE OVUM. 



between the two phases of the spindle PICRIC ACID PREPARATION 

 an intermediate one is found in which a f 60 ?'" 1 from Fo1 )- 

 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 



1 For the further details on the nuclear spindle vult the next Chapter. 



