Chap, xin.] HISTORY OF THE OVUM. 481 



germinal spot), brings to our mind the Amoeba, 

 with which our studies commenced ; and, if we observe 

 its early behaviour, we are the more struck with the 

 resemblance, for we often find it seizing on and 

 making part of itself the cells which surround it. In 

 the great majority of cases the cell becomes so far 

 differentiated that it develops around itself an in- 

 vesting (vitelliiie) membrane ; here, again, recalling 

 to mind the next stage in protozoic different iation in 

 so far as protoplasmic pseudopodial processes pass 

 through the pores in the membranous wall (Toxo- 

 pneustes). In more elaborated stages the surrounding 

 cells of the ovary give rise to more specialised 

 membranes, and in some cases it appears to be 

 necessary to leave an orifice (so-called " micropyle "), 

 by means of which nutrient material or fertilising 

 elements may be allowed to enter and come into 

 contact with the substance of the egg. 



The final act in the maturation of the ovum 

 appears to be the extrusion of the two polar 

 globules. The nucleus of the egg cell (the ger- 

 minal vesicle) moves towards the periphery of 

 the cell ; as it does so its surrounding membrane 

 becomes absorbed, and the contents altered in 

 character. What remains of the germinal vesicle 

 becomes spindle-shaped, and one end of the spindle is 

 protruded from the edge of the cell. The projecting 

 portion is next constricted from the rest, and so gives 

 rise to the first polar globule. The process is again 

 repeated, a second spindle being formed, and the 

 projection being again constricted oft" to give rise to 

 the second polar globule. 



Whatever be the real explanation of this pheno- 

 menon, it is, in the first place, clear that it bears 

 a very striking analogy to what happens in the 

 male cell, where a portion of the original protoplasm 

 becomes the inactive blastophor ; and we can hardly 



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