POLAR CELLS. 



FIG. 27. PORTION OF THE OVUM 

 OF ASTERIAS GLACIALIS, WITH THE 

 FIRST POLAR CELL AS IT APPEARS 

 WHEN LIVING (copied from Fol). 



this occurrence, which does not appear to take place in Hetero- 

 pods. 



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 con- 

 stricted 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 earli- 

 est to appear may be called the first 

 polar cell. 



The part of the spindle which re- 

 mains in the egg becomes directly con- 

 verted into a second spindle by the 

 elongation of its fibres, without pass- 

 ing through a typical nuclear con- 

 dition. A second polar cell next be- 

 comes formed in the same manner as 

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

 the spindle remaining in the egg be- 

 comes converted into two or three clear vesicles (fig. 29), which 

 soon unite to form a single nucleus (fig. 

 30). The new nucleus which is clearly 

 derived from part of the original germinal 

 vesicle is called the female pronucleus, 

 for reasons which will appear in the sequel. 



The two polar cells appear to be situ- 

 ated between two membranes, the outer 

 of which is very delicate, and only dis- 

 tinct where it covers the polar cells, while 

 the inner one is thicker and becomes, 

 after impregnation, more distinct, and 

 then forms what Fol speaks of as the 

 vitelline membrane. It is clear, as Hert- 

 wig has pointed out, that the polar bodies 



FIG. 28. PORTION OF THE 

 OVUM OF ASTERIAS GLACIALIS 

 IMMEDIATELY AFTER THE FOR- 

 MATION OF THE SECOND POLAR 

 CELL. PICRIC ACID PREPARA- 

 TION (copied from Fol). 



FIG. 29. PORTION OF 

 THE OVUM OF ASTERIAS 

 GLACIALIS AFTER THE FOR- 

 MATION OF THE SECOND 

 POLAR CELL, SHEWING THE 

 PART OF THE SPINDLE RE- 

 MAINING IN THE OVUM BE- 

 COMING CONVERTED INTO 

 TWO CLEAR VESICLES. PlC- 

 RIC ACID PREPARATION 

 (copied from Fol). 



