MATURATION OF THE EGG, AND PROCESS OF FERTILISATION. 37 



of the maturation of the egg were enveloped, has been in general cleared up, 

 the same is not the case if we inquire after its physiological meaning. That 

 the germinative vesicle undergoes a regressive metamorphosis into component 

 parts is easily comprehensible, for a firm membrane and a rich accumulation 

 of nucleoplasm certainly cannot be necessary to the interaction of protoplasm 

 and active nuclear substance in the processes of division. Its dissolution is, 

 as it were, the preliminary requirement for the renewed activity of the nuclear 

 contents. But what function shall one ascribe to the polar cells ? 



Concerning this several hypotheses have been proposed. 



BALFOUR, SEDGWICK MINOT, VAN BENEDEN," and others, are of opinion 

 that the immature egg, like every other cell, is originally hermaphroditic, and 

 that by the development of polar cells it rids itself of the male constituents of 

 its nucleus, which afterwards are replaced by fertilisation. BALFOUR thinks 

 that, if no polar cells were formed, parthenogenesis must normally occur. 



WEISMANN, supported by his discovery in the case of eggs developing 

 parthenogenetically (p. 34), ascribes a different function to the first and the 

 second polar cells. He distinguishes in the germinative vesicle two different 

 kinds of plasma, which he designates ovogenetic and germinal plasma. 

 He maintains that by the formation of the first polar cell the ovogenetic 

 plasma is eliminated from the ovum ; by that of the second polar cell, half 

 of the germinal plasma. In the latter case the ejected germinal plasma must 

 be replaced by fertilisation. 



These hypotheses appear to me upon closer examination to present many 

 vulnerable points. To me appears more promising an interpretation of 

 BUTSCHLI, who compares the egg, as had already often been done, to the 

 mother-cell of spermatozoa. Just as the latter gives rise to many spermatozoa, 

 so also the egg must have once possessed the capability of dividing itself into 

 many eggs. In the formation of the polar cells, which are eggs that have 

 become rudimentary, as it were, there has been preserved a trace of these 

 original conditions. Also BOVERI regards the polar cells as abortive egg*. 

 I have likewise always conceived of the conditions in this manner. 



2. The Process of Fertilisation. 



The union of egg-cell and spermatic cell is designated as the process 

 of fertilisation. This process is to be observed, sometimes with great 

 difficulty, sometimes with considerable ease, according to the choice of 

 the animal for experimentation. The investigator ordinarily en- 

 counters great difficulties in cases where the ripe eggs are not laid, but 

 where a part, if not the whole, of their development is effected within 

 the sexual ducts of the maternal organism. In such cases the fertili- 

 sation also must evidently take place in the ducts of the female sexual 

 apparatus, into which the semen is introduced in the act of 

 copulation. 



An internal fertilisation takes place in nearly all Vertebrates 

 except the greater part of the Fishes and many Amphibia. Usually the 

 egg and the spermatozoa meet, : in the case of Man and Mammals, in 



