58 



POLAR CELLS. 



FlG. 26. POETION OF THE 

 OVUM OF ASTEKIAS GLACIALIS 

 AT THE MOMENT OF THE DE- 

 TACHMENT OF THE FIRST POLAR 

 BODY AND THE WITHDRAWAL OF 

 THE REMAINING PART OF THE 

 SPINDLE WITHIN THE OVUM. 

 PlCRIC ACID PREPARATION 



(copied from Fol). 



of the fibres ("nuclear plate). In this way a complete nuclear spindle 

 is established . 



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, however, 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 this occurrence, which does not 

 appear to take place in Heteropods. 



In any case the spindle which 

 projects into the prominence on the 

 surface of the egg divides into two 

 parts, one in the prominence and one 

 in the egg (fig. 26). The prominence 

 itself with the enclosed portion of the 

 spindle becomes constricted off from 

 the egg to form a body, well known to 

 embryologists as the polar body or 

 cell (fig. 27). Since more than one polar cell is formed, that which 



is the earliest to appear may be called 

 the first polar cell. 



The part of the spindle which remains 

 in the egg becomes directly converted into 

 a second spindle by the elongation of its 

 fibres, without passing through a typical 

 nuclear condition. A second polar cell 

 next becomes formed in the same manner 

 as the first (fig. 28), and the portion of 

 the spindle remaining in the egg becomes 

 converted into two or three clear vesicles 

 (fig. 29), which soon unite to form a 

 single nucleus (fig. 30). The new nucleus 

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



* i 



FIG. 27. PORTION OF THE OVUM OF 

 ASTERIAS GLACIALIS, WITH THE FIKST 

 POLAR CELL AS IT APPEARS WHEN LIV- 

 ING (copied from Fol). 



FIG. 28. PORTION OFTHE OVUM 

 OF ASTERIAS GLACIALIS IMMEDI- 

 ATELY AFTER THE FORMATION OF 

 THE SECOND POLAR CELL. PlCRIC 

 ACID PREPARATION (copied from 



Fol). 



