Regeneration in ilydroniednsae 23 



mitotic figures, an explanation was sought for. As referred to 

 under methods, the hydroids were killed in Gilson's fluid or in a 

 mixture of corrosive acetic and picric acid. The killing was not 

 done with the particular aim of securing' mitotic figures, but 

 rather for general histologic investigation. It may be possible, 

 therefore, that the paucity of mitotic figures in most of the 

 species considered is due to improper killing, and that if methods 

 were employed especially to determine this phenomenon, mitosis 

 wound be fairly abundant. 



However, the presence of amitosis is a condition which de- 

 mands some explanation. In Tubularia crocea particularly the 

 occurrence of amitosis was very common, and in fact in all 

 the species studied it was more or less evident. Comparisons 

 of some of tb.e drawings will show nuclei in the process of di- 

 rect division. In fig. 63 this division of nucleus is being fol- 

 lowed by a division of the cAtoplasm. The rather common oc- 

 currence of this phenomenon and the relatively large amount 

 of evidence thus brought forward is very suggestive as to 

 amitosis being an active process in the regeneration of hydroids. 



Wilson (1896), in referring to amitosis says, "It is of ex- 

 treme rarity, if indeed it ever occurs in embryonic cells or such as 

 are in the course of rapid and continued multiplication. It is 

 frequent in cells . . . which are on the way toward degen- 

 eration." It has also been suggested that amitotic division may 

 involve the nuclei only, and the cytoplasm does not divide ; this 

 for the purpose of increasing the nuclear surface as an aid in 

 metabolism. Wilson further says, "In lower forms of life at 

 least (amitosis) does not necessarily mean the approach of de- 

 generation, but is the result of .special conditions." 



In the hydroids studied the amitotic nuclear division does not 

 seem always to be follojved by cytoplasmic division, though this 

 may of course be due to the too early stages of the division. 

 This suggests the possibility that the conditions here may be for 

 the purpose of increasing the nuclear sin-face to aid in metabolism. 

 Such an explanation might account for the presence of so many 

 amitotically dividing nuclei as are shown in fig. 65. 



Wilson refers to a paper by Ziegler (1896) in which it is 



297 



