ON THE INHIBITION BY ARTIFICIAL SECTION 



OF THE NORMAL FISSION PLANE 



IN STENOSTOMA. 



BY WILLIAM E. RITTER AND MISS EDNA M. CONGDON. 



Plate XVII. 



I. The Problem Suggested. 



The question presented itself to one of us (Ritter) a 

 number of years ago while studying monogenesis in com- 

 pound ascidians : What would be the result of artificial sec- 

 tion between two just-beginning normal fission planes in 

 the budding young of an Amarouciuni or a Pyrosoma f^ 



2. Selection of Animals for Experimentation. 



The difficulties in the way of carrying out the experiment 

 on these animals having proved to be great, efforts were 

 directed to the finding of some other creature that would 

 furnish essentially the same conditions and at the same 

 time would present fewer manipulative obstacles. 



Several species of naidiform worms of the genera Nais, 

 ChcBtogaster and Pristina were subjected to experimenta- 

 tion at first, but from these only negative results were 

 obtained; i. e., in spite of artificial division the normal fis- 

 sion proceeded as though no cut had been interposed, and 

 small pieces between the cut and the normal fission plane 

 were separated from the parent worm. It should be said, 

 however, that our experiments with these worms have been 



1 It will be recalled that in these, as well as in various other tunicates that repro- 

 duce asexually, the method, in essentials, is that the posterior portion of the young 

 animal developed from the egg divides up into pieces by transverse fission, each piece 

 giving origin to an ascidiozooid, which may, or may not, become entirely severed from 

 the parent zooid. See, for example, figures in most text-books taken from Kowalevsky, 

 Salensky, Brooks, etc. 



[365] December 26, 1900. 



