214 COMPARATIVE ANATOMY 



probable that it follows the course indicated in the diagram, 

 fig. 46. The young Vorticellid with four macronuclei under- 

 goes two more divisions, each accompanied by a division of 

 the micronucleus, whilst macronuclei are simply distributed 

 equally among the progeny. The young Vorticellid with three 

 macronuclei behaves similarly, but as there are only three 

 macronuclei to be distributed, only three young forms are pro- 

 duced in the manner shown in the diagram. 



The fate of the original macronuclei of the two gametes has 

 already been alluded to. At a very early stage they are broken 

 up by abstriction into a number of fragments, those of the 

 microgamete being always smaller than those of the macro- 

 gamete. The fragments continue to break up into smaller 

 and smaller pieces which are finally absorbed and disappear 

 altogether. 



One cannot fail to observe that the conjugation of Vorticella 

 is a step further advanced towards a differentiation of sex than 

 in Paramecium, or^ indeed, in any other of the unequivocally 

 animal Protozoa previously described in these pages. Volvox 

 exhibits a still more marked sexual differentiation, but, whilst 

 it is allied to animal forms, it has all the characters of a plant. 

 Whereas in Paramecium the gametes are similar, the conjuga- 

 tion temporary, and both of the ex-gametes reproduce their 

 kind, in Vorticella the gametes are dissimilar, the conjugation 

 permanent and complete, and the resulting zygote is the sole 

 reproductive individual. We cannot hesitate to ascribe to the 

 active microgamete the role of the male, to the passive macro- 

 gamete the role of the female. 



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