PROBLEMS AND MATERIAL 13 



from certain parts of the body. In some forms the spore is apparently a 

 more highly differentiated cell than other body cells, and in many plants 

 it is formed only in highly differentiated organs; yet it has the potentiality 

 of developing into a new multicellular individual. Multicellular spore- 

 like bodies — sponge gemmules, bryozoan statoblasts, and some ascidian 

 winter buds — have a different past history from that of the egg but can 

 develop into the same sort of individual. 



In cases of fission which give rise to new individuals, the reproductive 

 body has previously been merely a part of the parent body. As such it has 

 had a past history of development and more or less differentiation; more- 

 over, it is not always the same part of the parent body which is separated 

 by fission, but the kind of individual which develops from it does not de- 

 pend on this differentiation. In the fiatworms and annelids which under- 

 go fission the course of developmental reorganization may differ according 

 to the body region separated by fission, but similar individuals result. 

 Origin of segments seems to be essentially a repeated fission, primarily 

 mesodermal, with limited development of each. Segments are formed in 

 embryonic development, in the development of zooids and of isolated 

 pieces of segmented animals, but the manner in which they arise differs 

 somewhat in the different forms of development, though the final result 

 may be the same. 



Experimental isolation of pieces of the individual by section, with fol- 

 lowing reconstitutional development, is purely accidental, as far as the 

 original individual is concerned. The different parts have had different 

 past histories and undergo different courses of reorganization in giving 

 rise to similar individuals. Also, they may be of widely different size and 

 develop into individuals of different size ; and the animals from which they 

 are taken may be of different age, nutritive condition, etc. Such forms of 

 development provide much more favorable material for various lines of 

 physiological analysis than does embryonic development. 



Various multicellular axiate algae separate into individual cells under 

 certain unfavorable conditions, and these cells are capable of developing 

 into new axiate multicellular individuals.'' Certain planarian species, if 

 kept above a certain temperature, separate into fragments when they at- 

 tain a certain size, the size attained and the size of the fragments varying 

 with the character of nutrition (Child," 1913c, igi^d). At low temperature 

 they become sexually mature instead of fragmenting (Castle, 1928). Cer- 

 tain nemertean species fragment on stimulation. In the planarian frag- 



^ Tobler, 1902, 1904, 1906; Child, igijb. 



