666 LECTURE XXIII. 



this: that in their development the necessary exclusion of 

 a portion of the protoplasmic contents of the oogonium does 

 not take place. 



We may digress for a moment to consider a form of 

 parthenogenesis, occurring among plants in which sexual 

 differentiation is comparatively rudimentary, namely, in Botry- 

 dium, Ectocarpus, and Ulothrix, which differs from that of 

 the Saprolegnieae and therefore merits special consideration. 

 With regard to Botrydium there can be no doubt that the 

 motile reproductive cells produced from the resting-spore 

 are morphologically gametes, but they are only physiologi- 

 cally gametes provided that the resting-spore is young ; 

 when the resting-spore is old, the cells to which it gives 

 rise are simply zoospores (see p. 607). In the absence of 

 any evidence to shew that the process of development of 

 the motile reproductive cells is different in a young and in 

 an old resting-spore, we can only explain the facts by 

 assuming that during the prolonged quiescence of the resting- 

 spore, changes take place within it the result of which is that 

 it is converted from a sexual into an asexual reproductive 

 organ ; the changes being probably of this nature, that the 

 amount of the nucleo-idioplasm increases, so that the cells 

 eventually produced contain sufficient nucleo-idioplasm to 

 enable them to germinate independently. The case of 

 Ectocarpus and of Ulothrix is somewhat different. Here 

 the reproductive cells are gametes when they are first formed, 

 but if they fail to conjugate they are capable of independent 

 germination. In this case, possibly, an increase of the nucleo- 

 idioplasm to the degree necessary to permit of independent 

 germination takes place in the reproductive cells themselves. 



But to return. Weismann strongly opposes the view that 

 in the extrusion of the polar body any portion of the repro- 

 ductive substance, or nucleo-idioplasm, is thrown off: he 

 considers that, as already mentioned, the extrusion of the 

 polar body is simply the elimination of the histogenic nucleo- 

 plasm. If this be admitted, then it follows that the process 

 of development of a gamete does not differ essentially from 

 that of a spore. This is just the position which Weismann 



