LEONARD P. SAYLES. 



oval measuring about 3.5 x 6 // and contain nucleoli less than 

 i n in diameter. The nuclei of these gland cells, however, have 

 not decreased in size but are about the same as those of the 

 enlarged ectoderm cells of the dorsal and lateral regions (Fig. 12). 

 The average measurement of twenty-five of these is 5.2 x 7.3 /j. 

 with nucleoli 1.8 IJL in diameter. Occasionally double nucleoli are 

 to be found. The fact that these cells push into the ccelom 

 offers another point of confusion which may have led early 

 workers to think that the ectoderm migrated into the ccelom to 

 form the new mesodermal structures. 



It will be seen that in all three types of cells (endodermal, 

 mesodermal and ectodermal) which take part in the formation 

 of new tissue at the posterior end there is one outstanding 

 feature in common. The nuclei and nucleoli enlarge. The 

 greatest changes occur in the neoblasts and in the cells of the 

 ventral portion of the hypodermis. In both of these types 

 the nuclei of the fully transformed cells are eight or nine times 

 as large as ordinarily. The nucleoli increase even more in 

 proportion, particularly in the hypodermal cells in which they 

 enlarge from mere dots to structures over 3 // in diameter. 

 Even in the gut cells where the nucleoli are usually of considerable 

 size there is probably a ten-fold increase. The significance of 

 these facts will be discussed later. 



ORIGIN OF NEW TISSUE IN ANTERIOR REGENERATION. 



Endoderm. 



In anterior regeneration as in posterior regeneration the 

 majority of investigators Rievel ('96), Haase ('98), von Wagner 

 ('oo), Iwanow ('03) and Krecker ('10) are agreed that most of 

 the intestine of the regenerating bud is formed by the growth 

 of that in the old part. This formation of new tissue involves 

 about eleven or twelve segments here as in posterior regeneration. 

 The nucleoli are considerably enlarged at the end of the first 

 day and double nucleoli are fairly common. Only a few mitoses, 

 however, are present. The numbers both of double nucleoli and 

 of mitoses reach a maximum between the second and third days. 

 On the fourth day both are fewer in number in the more distant 

 segments. In the case of double nucleoli this decrease is partially 



