218 . CORA J. BECKWITH 



development. Hydroid planulae are hard to carry beyond this 

 point of development in the laboratory, since any disturbance 

 prevents the planula from attaching, which is essential for further 

 development. Even so slight a disturbance as changing the water 

 in the dish is sufficient to prevent attachment, so that further 

 development after so great a disturbance as separating the blas- 

 tomeres is impossible. 



The above experiments are of value only in indicating that the 

 mitochondria are not essential for differentation as far as the 

 planula-stage and that they can hardly be vital constituents of 

 the protoplasm since they may be centrifuged out of the proto- 

 plasm like any metaplasmic body, such as yolk. 



2. Discussion 



Mitochondria in Hydractinia do not agree in a number of points 

 with descriptions of these bodies in other forms. An extensive 

 review of the subject will not be attempted, however, since it 

 has been so well discussed and reviewed by many recent investi- 

 gators (Faure-Fremiet '10, Prenant '10 b, Montgomery '11, 

 Duesberg '11). Duesberg presents a monumental review of 

 the literature on the subject in which he brings together in classi- 

 fied form and in the most exhaustive way, all papers concerned 

 with mitochondria and chromidia. The opposing views as to 

 the nuclear or protoplasmic nature of these bodies have already 

 been stated. The above writers assume the identity of mito- 

 chondria and chromidia. Schaxel, on the other hand, considers 

 them quite distinct, as has been sufficiently indicated above. It 

 is also apparent from the aforegoing description that Hydractinia 

 corresponds with Schaxel's observations in this respect, that is, 

 that two distinct elements are present, mitochondria of undoubted 

 protoplasmic origin and basophilic granules in the protoplasm. 

 The latter, however, are not chromatic in Hydractinia and there- 

 fore not chromidia (extra-nuclear chromatin of Schaxel). 



I also agree with Schaxel in finding that the pseudochromatin- 

 granules first appear in the early growth-period of the egg, thus 

 giving no evidence of continuity from cell generation to generation. 



