FERTILIZATION OF THE OVUM 



maternal chromatophores lie at opposite ends. In .S. \Vcbcri, on 

 the other hand, Overton has found that the single maternal chromato- 

 phore breaks in two in the middle and the paternal chromatophore 

 is interpolated between the two halves, so as to lie in the middle of 

 the zygote (Fig. 85). It follows from this, as De Vries has pointed 

 out, that the origin of the chromatophores in the daughter-cells 

 differs in the two species, for in the former case one receives a 

 maternal, the other a paternal, chromatophore, while in the latter, 

 the chromatophore of each daughter-cell is equally derived from 

 those of the two gametes. The final result is, however, the same ; 

 for, in both cases, the chromatophore of the zygote divides in the 

 middle at each ensuing division. In the first case, therefore, the 

 maternal chromatophore passes into one, the paternal into the other, 

 of the daughter-cells. In the second case the same result is effected 

 by two succeeding divisions, the two middle-cells of the four-celled 

 band receiving paternal, the two end-cells maternal, chromatophores. 

 In the case of a Spirogyra filament having a single chromatophore 

 it is therefore "wholly immaterial whether the individual cells re- 

 ceive the chlorophyll-band from the father or the mother " (De Vries), 

 a result which, as Wheeler has pointed out, is in a measure analo- 

 gous to that reached in the case of the centrosome of the animal egg. 1 



F. SUMMARY AND CONCLUSION 



All forms of fertilization involve a conjugation of cells by a 

 process that is the exact converse of cell-division. In the lowest 

 forms, such as the unicellular algae, the conjugating cells are, in a 

 morphological sense, precisely equivalent, and conjugation takes 

 place between corresponding elements, nucleus uniting with nucleus, 

 cell-body with cell-body, and even, in some cases, plastid with plastid. 

 Whether this is true of the centrosomes is not known, but in the 

 Infusoria there is a conjugation of the achromatic spindles which 

 certainly points to a union of the centrosomes or their equivalents. 

 As we rise in the scale, the conjugating cells diverge more and more, 

 until in the higher plants and animals they differ widely not only 

 in form and size, but also in their internal structure, and to such an 

 extent that they are no longer equivalent either morphologically or 

 physiologically. Both in animals and in plants the paternal germ- 

 cell loses most of its cytoplasm, the main bulk of which, and hence 

 the main body of the embryo, is now supplied by the egg. But, 



1 De Vries's conclusion is, however, not entirely certain; for it is impossible to deter- 

 mine, save by analogy, whether the chromatophores maintain their individuality in the 

 zygote. 



