776 LECTURE XLIII. 



is now held fast and cannot go back again : a sort of convulsion then foUow's/ 

 and the antherozoid touches the oosphere with its anterior end. At the moment this 

 contact of the two sexual cells occurs, the oosphere turns pale at the point of 

 contact and there appears a somewhat large rounded colourless and bright spot-^ 

 the receptive spot — which is however somewhat translucent at the edges, and this 

 indicates the place of fusion of the sexual cells : its appearance and occurrence 

 in this way have not yet been observed in other plants. Immediately after the 

 appearance of the receptive spot, the union of the antherozoid with the substance of 

 the oosphere begins, a vigorous contraction of the latter and the slow gliding in of the 

 former being very clearly observable. The contraction of the oosphere is so vigorous 

 that its change of form thus produced induces at the same time a change of form in 

 the oogonium also. 



'The slipping in of the antherozoid through the narrow opening of the neck 

 of the oogonium, and its union with the female sexual cell remind the observer 

 Involuntarily and in the most forcible manner of those phenomena which have been 

 observed in the conjugation of Spirogyra, for instance ; since here, as there, we see 

 that the fertilizing sexual cell, in order to be able to unite with the one to be 

 fertilised must make its way through an opening which is disproportionately narrow 

 in relation to its size, and accommodate its form and size to this opening — here as 

 there contraction of the sexual cells takes place before and during their union, and 

 just as clearly also, on account of the striking difference in colour and size of the 

 antherozoids their fusion with the substance of the oosphere may be followed in the 

 clearest manner.' 



Finally may be given the description afforded by De Bary and Strasburger 

 {hot. Zeit. 1877, P- 74^) °f 'he behaviour of the sexual swarm-spores or gametes of 

 Acetabularia, a non-cellular marine Alga : — 



'In spite of the fact that the drops used for observation appeared to form a 

 thoroughly efficient medium for the development and movement of the swarmers, 

 I nevertheless saw that at first all the swarmers perished, without one of them having 

 germinated. In doing this the swarmers rounded themselves off, formed vacuoles in 

 their interior, their chlorophyll-grains became disorganised and emptied of starch- 

 grains, and soon the whole resembled an amoeba of indefinite outline, which 

 finally resolved itself into a granular spot. 



' The movement of the swarmers in this case continued for twenty-four hours 

 in the most favourable cases, and I saw swarmers which remained in the 

 interior of unopened spores often still moving even after forty-eight hours. As 

 a rule a tendency to disorganisation began after a few hours, however, in some 

 cases after a few minutes. 



' The circumstance that I now also saw two spores, which from the appear- 

 ance of their vesicles had probably opened simultaneously, suggested to me that 

 the copulation only takes place between swarmers of different origin. Several 

 weeks then passed till the fortunate accident which showed me what is now known. 

 It happened eventually on a very sunny and warm day that numerous spores, 

 favoured by previous warm weather, emitted their contents. 



' I saw about mid-day two neighbouring spores, utterly indistinguishable from 

 one another, open under my eyes, and the swarmers of both hurry straight to the 



