REPRODUCTION. 659 



is to be accounted for thus, that almost all parts of plants 

 contain embryonic cells, or that their cells are capable of 

 readily returning to the embryonic condition : in other words, 

 the nuclei of certain cells in these parts are either already 

 rich in nucleo-idioplasm, or are capable of becoming so. 



In harmony with this conception, Strasburger regards the 

 extrusion of the paranucleolus in the development of spores, 

 and of the polar body in the development of gametes, as 

 being the expression of the return of the corresponding nuclei 

 to the embryonic condition: but he does not consider that 

 the extrusion of a polar body is an essential condition of the 

 development of a gamete. He says, namely, that the differ- 

 entiation of the nucleus of the male or the female gamete 

 does not depend upon the extrusion of definite constituents, 

 but upon a rearrangement of its substance, this rearrange- 

 ment being, however, accompanied in many cases by an 

 extrusion of a portion of the nuclear substance. In his 

 opinion the extrusion of a portion of the nuclear substance 

 does not take place in the majority of cases, but only an 

 excretion or a delimitation of a portion of the cytoplasm, 

 which has the effect of ensuring the appropriate nutrition 

 of the nucleus by the remaining cytoplasm. On another page 

 of his work, however, Strasburger seems to take a somewhat 

 different view, for he insists there that the differentiation 

 of generative nuclei depends upon their idioplasm being 

 reduced to one half the mass of that in a fertilised female 

 cell. This reduction is affected by indirect nuclear division. 

 Hence, the two nuclei are assumed to be exactly alike, so 

 that the extrusion of a polar body does not mean the ex- 

 trusion of particular constituents of the nucleus but simply its 

 reduction by one half. When, as in the pollen-tube, the two 

 nuclei thus formed clearly differ in function, Strasburger 

 accounts for it by assuming that the nuclei, though quite 

 similar at first, have come to differ in consequence of having 

 been somewhat differently nourished. 



Strasburger is not disposed to admit that there is any 

 essential difference between the sexual reproductive cells. 

 He considers that two gametes, for instance an oosphere 



42 2 



