DEVELOPMENT IN PLANARIANS. 287 



The destruction of the genital apparatus in isolated tail pieces 

 shows that in the reorganization which takes place during regen- 

 erative development highly differentiated tissues, like the mus- 

 culature of the genitals, are not transformed into new parts, but 

 instead undergo retrograde metamorphosis and serve probably as 

 food stuff for the other tissues. On the other hand, highly or- 

 ganized parts may be utilized directly in the formation of new 

 organs. This is shown by the union of a tip of the pharynx re- 

 tained in the pharyngeal pocket to a regenerated base. 



SUMMARY. 



1. The anlage of the pharynx is formed at one extremity of 

 the yolk-cavity of the embryo, that of the head at the other. 



2. Up to the period of the formation of the pharynx the yolk- 

 cavity is surrounded by a layer of parenchymal cells and these 

 by a thin epithelial sheet. The parenchymal cells multiply by 

 direct and by indirect division and become reduced in size as they 

 increase in number. 



3. The ectodermal cells begin to multiply rapidly by direct di- 

 vision after the pharynx has been formed, and thus give rise to 

 a columnar epithelium. The dorsal cells at no time develop 

 cilia, while the ventral cells produce them before becoming co- 

 lumnar in form. 



4. Tissue differentiation begins in the parenchyma shortly be- 

 fore the pharyngeal anlage appears. From the parenchyma are 

 developed intestinal cells, muscle cells, nerve cells, rhabdite cells 

 and gland cells, as well as branched connective-tissue cells and 

 possibly leucocytes. Certain of the parenchyma cells seem to 

 multiply without undergoing further differentiation. Intestinal, 

 nerve and muscle cells multiply by direct division, while the 

 parenchyma cells multiply mainly by indirect division. 



5. The central nervous system is first developed in the region 

 of the head and from there the lateral cords grow posteriorly. 



6. When removed from the cocoon very young embryos soon 

 die. Slightly older ones die if injured. After the pharynx has 

 become functional the embryos may be cut into two or more 

 parts which will live. Pieces from which the head has been 

 removed will not develop a new head unless they contain well- 

 developed nerve cords. 



