44 THE STRUCTURE OF CELLS 



if anywhere, are to be sought the clues to the complex though 

 less variable processes which characterise the higher animals and 

 plants. 



The phenomena of the sexual union of germ cells, and of their 

 contained nuclei, for the most part are, as yet, hardly susceptible of 

 detailed explanation, but there can be no doubt but that chemiotaxis 

 is the proximate factor chiefly concerned. This is beautifully shown 

 in the case of Halidrys, one of the Fucaceae, in which the large 

 eggs attract numbers of sperms which seek to penetrate the egg. 

 Immediately after an entrance has been effected by one of them, it 

 is seen that the egg changes in important respects. It shrinks, 

 and the supernumerary sperms instantly cease their endeavours to 

 enter its substance. On the contrary, they swim rapidly away, 

 and from the surface of the egg a substance is seen to be excreted 

 which probably exerts the repellent influence in question. Indeed, 

 so strong is its action that those sperms which have not quitted 

 the surface of the eggs are rapidly paralysed and killed. 



The remarkable paths followed by the male nucleus in the egg 

 has been studied by many observers, and there can be but little 

 doubt that here also a specific attraction of some sort effects the 

 final union. As to the significance of the fusion, the evidence at 

 present available points in no certain direction, nor can any of the 

 hypotheses which have as yet been advanced to explain it be 

 regarded as affording satisfactory solutions of the problem. It has 

 been assumed that by its means a sort of rejuvenescence is effected. 

 But this idea, which is not very clear in itself, fails to take the 

 many subsidiary but still general recurring circumstances into con- 

 sideration. Moreover, it is difficult to see why a similar explana- 

 tion should not also cover those vegetative fusions common in 

 endosperm cells and in certain fungal hyphae, but these have never 

 been regarded as constituting sexual acts. 



And indeed it would appear that the actual initiation of segment- 

 ation in an egg is not necessarily dependent on the fusion or even 

 the presence of two nuclei. Boveri's observations on the fertilisa- 

 tion of enucleated fragments of eggs with sperms, and still more 

 those of Loeb, who succeeded in causing the eggs of sea-urchins to 

 segment parthenogenetically by treating them with magnesium 

 chloride, indicate that the matter is of far greater complexity than 

 a study of the normal occurrences would indicate. Again, Nathan- 

 sohn caused parthenogenetic development of the oospheres of 

 Marsilea to take place by keeping them at a sufficiently high 

 temperature. 



This last observation seems to be one of greater importance, for 

 it suggests that a slight modification of the metabolic processes, 

 in this case effected by the abnormal temperature, may suffice to 

 set the machinery of segmentation in motion ; that is, the actual 



