CHAPTER XV 

 TWINNING AS A MODE OF REPRODUCTION 



In view of the confusion that prevails at present 

 regarding twinning processes I believe that the following 

 attempt to classify the various modes of reproduction, 

 including the several types of twinning, will help to 

 clarify the situation. 



One school of writers finds the broadest distinction 

 among modes of reproduction to lie in whether they are 

 sexual or asexual. All types of reproduction in which 

 there is no union of gametes are lumped together into 

 the category of agametic or asexual reproduction as 

 over against gametic or sexual reproduction. 



In his textbook, Principles of Zoology, Shull (1920) 

 expresses himself as follows : 



Asexual or non-sexual reproduction includes all those methods 

 of reproduction which require but a single parent for the reproduc- 

 tion of offspring and do not involve germ cells. Sexual reproduc- 

 tion as a rule involves two parents and the production of two kinds 

 of germ cells, the eggs and sperms. It is usually brought about 

 by union of a sperm cell with an egg, or less commonly by the 

 development of the egg without union with the sperm. 



He then proceeds to classify modes of reproduction 



essentially as follows: 



I. Asexual reproduction 



N Ti J J- [external 

 a) Buddmg|i^^^^^^i 



,s -I.. . /longitudinal 

 h) Fission j^^^^^^^^^g^ 



c) Sporulation 



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