THE EGG-CELL OR OVUM. 107 



{c) Weismann's view is different from either of the above, though 

 nearer the first. He distinguishes in the nucleus of the ovum two kinds of 

 plasma, — (i) the ovogenetic or histogenetic substance, which enables the 

 ovum to accumulate yolk, secrete membranes, and the like ; and (2) the 

 germ-plasma, which enables the ovum to develop into an embryo. When 

 the ovum is mature, the ovogenetic substance has served its turn ; it is 

 henceforth only an encumbrance ; it is extruded as the first polar globule. 

 This is all that is extruded in parthenogenetic ova. The second extrusion 

 is a reduction of the germ-plasma itself by half, and the same must occur 

 in the male germ cell too. What is lost in the second polar globule is 

 supplied by the fertilising sperm. The beginning of development depends 

 upon the presence of a definite quantity of germ-plasma. This the normal 

 egg attains by first losing half and then regaining it, while the partheno- 

 genetic egg attains the same result by never losing any at all. 



In this too there is much hypothesis. The two kinds of nuclear plasma, 

 the difference between the two polar globules, the necessity for a definite 

 quantity before development begin, are all assumptions. Nor is it at all 

 evident how the advantage of fertilisation (as a source of progressive change 

 and so on) could operate, so as to induce the ovum to go through the circuitous 

 process of losing half its " germ-plasma," and then gaining it again. 



(d) It appears simpler to us to suppose that the ovum, like any other cell, 

 tends to divide or bud at the limit of growth, a view in no way inconsistent 

 with regarding the process as an extrusion of male elements. The precise 

 homologies of the process will be clearer on reference to the diagram at 

 page 114. 



