60 



COMPARATIVE PHYSIOLOGY. 



The germinal vesicle may acquire powers of slow movement 

 (amoeboid), and the germinal spot disappear : the former passes 

 to one surface {pole) of the ovum ; both these structures may 

 undergo that peculiar form of rearrangement (karyohinesis) 

 which may occur in the nuclei and nucleoli of other cells prior 

 to division ; in other words, the ovum has features common to it 

 and many other cells in that early stage which precedes the com- 

 plicated transformations which constitute the future history of 

 the ovum. 



A portion of the changed nucleus {aster) with some of the 

 protoplasm of the cell accumulates at one surface {pole), which 

 is termed the upper pole because it is at this region that the epithe- 

 lial cells will be ultimately developed, and is separated. This pro- 

 cess is repeated. These bodies {polar cells, polar globules, etc.), 



Fig. 59.— Ponnation of polar cells in a star-flsh (Asierias glacialis) (from Geddes, 

 A — ^K after Fol, L after O. Hertwig). A, ripe ovum with eccentric germinal vesi- 

 cle and spot; B— D, gradual metamorphosis of germinal vesicle and spot, as seen 

 in the livmg egg, into two asters: F, formation of first polar cells and withdrawal 

 of remaining part of nuclear spindle within the ovum; G, surface view of living 

 ovum in the first polar cell; H, completion of second polar cell; I, a later stage, 

 showing the remaining internal half of the spindle in the form of two clear vesi- 



cles; K, ovum with two polar cells and radial striae round female pronucleus, as 



:_ »i,. i;..: (E. F, H, aud ' * "-'" —'■' " '" ^ '"' — 



(Haddon.) 



seen in the living egg (E. F, H, and I from picric acid preparations); L, expulsion 

 of the first polar celH '" " ' - 



then, are simply expelled ; they take no part in the development 

 of the ovum ; and their extrusion is to be regarded as a prepar- 

 ation for the progress of the cell, whether this event follows or 

 precedes the entrance of the male cell into the ovum. It is wor- 

 thy of note that the ovum may become amceboid in the region 

 from which the polar globules are expelled. 



The remainder of the ■amilett.s{ female pronucleus) now passes 

 inward to undergo further changes of undoubted importance, 

 possibly those by virtue of which all the subsequent evolution 

 of the ovum is determined. This brings us to the consideration 

 of another cell destined to play a brief but important rdle on the 

 biological stage. 



