88 FECUNDATION; HETEROGAMETES. 



an unrecognizable condition. In the light of what is known in certain 

 animal eggs such an inference was tempting, but, from our present 

 knowledge of the centrosphere and centrosome in plants, such a con- 

 clusion is no longer justifiable. Moreover, when the centrospheres 

 appear in the first nuclear division of the fecundated egg, it is difficult, 

 and may be impracticable, to distinguish between the male and female 

 portions. 



Only in rare cases does more than one spermatozoid enter the egg, 

 for among several thousand preparations examined by Farmer and 

 Williams, only three cases of polyspermy were observed in which two 

 spermatozoids had effected an entrance. The rare occurrence of poly- 

 spermy under such conditions as are normal for the plants concerned, 

 and as appears favorable for this phenomenon, would seem to indicate 

 that many cases of polyspermy reported for animals might be largely 

 the result of the prevalence of abnormal conditions at the time of 

 fecundation. 



Concerning the large oosphere-like bodies with two nuclei in Fucus, 

 which have been regarded by Behrens as fecundation stages, the joint 

 authors cited above state with emphasis that these " represent either 

 abnormally developed oospheres or oogonia." 



VOLVOX. 



Without implying any relationship whatever between the two groups 

 of plants to which they belong, the sexual process in Volvox may be 

 fittingly mentioned along with that of Fucus. In this most highly- 

 differentiated representative of the Volvocacece we have highly special- 

 ized sexual cells, and in fact, as has been already stated in a preceding 

 chapter, there is in this group of plants, as in the brown algae, a 

 gradual transition from the simplest form of sexual reproduction of 

 isogametes to that of the well differentiated bisexual elements of Volvox. 



Some authors (Strasburger, '92, 1900; Overtoil, '89) regard the 

 spermatozoid of Volvox as a transition between the motile isogametes 

 of algae and the spermatozoids of the Characeae. The spermatozoid 

 of Volvox glob ator tapers gradually to a slender anterior end which is 

 colorless, the thicker posterior end being yellowish. At the boundary 

 between the two lies the red eye-spot, and a little farther forward are 

 borne the two laterally inserted cilia. It is reasonable to assume that 

 the cilia spring from a blepharoplast, although positive proof is still 

 wanting. Strasburger (1900, p. 196) regards the colorless and slender 

 anterior end as the homolog of the mouth-piece of algal gametes, from 

 which such highly differentiated bisexual elements as those of Volvox 



