275 



G. T. Hargitt ('03) was the first so far as I am aware to note 

 the frequent occurrence of amitosis during regulation in hydroids. He 

 believes that the current opinions regarding amitosis do not suffice 

 to explain its frequency in these forms. 



Plathelminthes. 



Plana ria maculata. In 1901 I observed frequent cases of 

 apparent amitosis in the regenerating tissues of Planaria, but post- 

 poned publication until further work could be done. Somewhat later 

 Bardeen ('02) noted the frequency of amitosis in the normal devel- 

 opment and regeneration in this species and did not hesitate to assert 

 that it gave rise to new cells. He also found as did Flexnek ('98) 

 that mitosis was of frequent occurrence. 



According to later observations of my own in specimens under- 

 going regulation mitosis appears a few hours after the operation in 

 the regions adjoining the cut surface. After about twenty-four hours, 

 however, mitosis is no longer found in this region but frequent ami- 

 toses occur in the new tissue. But regulation in Planaria is in part 

 a process of redifferentiation. If, for example, half or two thirds of 

 the post-pharyngeal region is removed, the remaining portion of the 

 post-pharyngeal region undergoes a considerable increase in size and 

 some changes in structure. This redifferentiation is a much slower 

 process than the outgrowth of new tissue, except where new organs 

 like the pharynx arise. Mitosis is of frequent occurrence, but is not 

 the sole method of division, in the redifi"erentiating region during re- 

 gulation, and is very rare or absent elsewhere. In the region of the 

 new pharynx it appears in early stages but is later replaced by ami- 

 tosis. So far as my observations go, mitosis, when it occurs in Planaria, 

 appears to be confined chiefly or wholly to the larger cells in the 

 parenchyma which possess a considerable amount of cytoplasm. In all 

 normal specimens of Planaria examined, mitoses are exceedingly rare 

 in any region, even when the animals are well fed and growing rapidly, 

 but amitoses are very abundant. I am inclined to believe that the 

 cells which divide mitotically during regulation have been taking little 

 or no part in the normal proliferation and so have grown to a larger 

 size than the others. But with the increased intensity of the stimulus 

 to growth which follows removal of a part these cells undergo mitotic 

 division and later in those regions where the stimulus to growth is 

 most intense they begin to divide amitotically. 



In Figure 3 a number of cases of apparent amitosis are shown. 

 Nos. I — VI are from the regenerating posterior end, some being muscle 



18* 



