SUMMARY AND CONCLUSION 229 



unite, but in many cases the plastids also (chromatophores). In 

 Spirogyra some interesting variations in this regard have been ob- 

 served. In some species De Bary has observed that the long band- 

 shaped chromatophores unite end to end so that in the zygote the 

 paternal and maternal chromatophores lie at opposite ends. In 

 S. Weberi, on the other hand, Overton has found that the single 

 maternal chromatophore 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. 113). 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, chro- 

 matophores. In the case of a Spirogyra filament having a single 

 chromatophore it is therefore "wholly immaterial whether the indi- 

 vidual cells receive the chlorophyll-band from the father or the 

 mother" (De Vries).^ 



F. Summary and Conclusion 



All forms of fertiHzation 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- 



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. 



