CHAPTER X 



THE ORIGIN OF NEW CELLS AND TISSUES 



THERE are many difficulties in the way of determining the origin 

 of the cells that make up the new part. The only means at present 

 at our command for studying their source is by serial sections of a 

 number of different stages taken at intervals from different animals. 

 Since there may be differences between the processes in different in- 

 dividuals, and since we can only piece together the information gained 

 from successive stages, much uncertainty exists in regard to the 

 changes that take place during regeneration, even in some of those 

 forms that have been examined over and over again. Were it possi- 

 ble actually to follow out the movements of the living cells in one and 

 the same animal, the problem would offer fewer difficulties, but this 

 cannot be done. It will be more profitable to consider first the bet- 

 ter-known and simpler processes, and afterward those that are less 

 well-known. 



The regeneration of the head and tail of lumbriculus and of cer- 

 tain naids is a comparatively simple process, and has been studied by 

 several investigators, whose results agree, at least in regard to the most 

 essential features. Semper ('76) described the origin of the new 

 organs in the formation of new individuals by budding in nais. He 

 found that the new brain and nerve-cord develop from the ectoderm, 

 the newmesoderm also from ectoderm, and the new digestive tract from 

 the old one, except the pharynx, which arises by the fusion of two meso- 

 dermal " gill-slits." Bulow ('83) studied the regeneration of the tail of 

 lumbriculus. He found the ventral cord in the new part arising from a 

 paired ectodermal thickening, the mesoderm arising from a prolifera- 

 tion of cells. These cells are invaginated in the region between ecto- 

 derm and endoderm the in-turning of the proctodaeum being looked 

 upon as an endodermal invagination. 1 The more recent work of Ran- 

 dolph, Rievel, Michel, Hasse, Hepke, and von Wagner on the same or 

 related forms has served to point out certain errors in the earlier work 

 of Semper and Billow, and has added some new and important facts, 

 especially in connection with the origin of the mesoderm in the new 

 part. Without attempting to give a detailed account of these results, 



1 The usual interpretation at present is to regard the proctodseal ingrowth as ectodermal. 



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