THE BIOLOGY OF THE CELL SURFACE 



that with respect to even simpler material structure nature 

 does not always limit itself to one mode, that we must 

 amend our natural laws as we learn more about nature. 

 Such discoveries also suggest that more complex natural 

 processes wear many guises. Had the parthenogenesis 

 that occurs in nature received due attention — if in fact it 

 had come within the knowledge of those who so greatly 

 busied themselves with experimental parthenogenesis — the 

 discovery of experimental parthenogenesis never would 

 have had the palm awarded it. 



Parthenogenetic development is found naturally occur- 

 ring especially in the groups of rotifers and arthropods. 

 This natural parthenogenesis may be fixed (obligatory) — 

 i.e., the eggs develop normally only parthenogenetically. 

 Experimentally, a change either in the environment or 

 of the food of the mother may render the eggs capable of 

 receiving the spermatozoon. In such cases the period of 

 fertilizability comes during first maturation. Natural 

 parthenogenesis is also variable (facultative) — these eggs 

 develop normally either parthenogenetically or by fertiliza- 

 tion: as, for example, eggs of the honey-bee. 



In fixed parthenogenesis, the first polar body is extruded, 

 the second is not; hence the egg develops with the double 

 (somatic) number of chromosomes. The chromosomes of 

 the second polar body may unite with those of the egg- 

 nucleus, thus acting as a substitute for the sperm-nucleus.^ 

 An egg of the variable type of parthenogenesis extrudes 

 both polar bodies and develops with only a single set of 

 chromosomes. A double set of chromosomes is not, there- 

 fore, a prerequisite for the onset of naturally occurring 

 parthenogenetic development; nor does the presence of 

 only a single set of chromosomes imply defective develop- 

 ment: the male honey bee (or drone) for instance, partheno- 



^ Cf. Artemia, Brauer, /S^j. 



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