284 



of regenerating lost parts, and this capacity has, naturally, been 



referred back to the existence of embryonic tissue in the hydroid. 

 The adjacent figure represents the distal end of a stock of Obelia 



commissuralis — one of the commonest hydroids on our New Eng- 

 land coast. The stem of the individual B, whose 

 axis is represented by the broken line, may be 

 cut at any niveau, as at «, ß, or y, and rege- 

 neration of the hydroid individual will occur. This 

 regeneration must be due to the presence of em- 

 bryonic tissue at the levels a, /?, y respectively. 

 The question now arises, is the embryonic tissue 

 qualitatively the same at all these levels or is it 

 different, so that what lies at the level a has 

 Fig i the potentiality of producing the entire hydroid 



individual , that lying at the level ß only the 



hydranth with a ring or two of the stalk. The answer to this question 



must evidently be obtained through experiment. 



II. Methods. 



The species mentioned is particularly favorable for answering the 

 question, because of the fact that it is easy, on account of the un- 

 equal segmentation of the stem, to define readily and accurately the 

 position of the lines and to give a numerical expression to the result 

 of regeneration after a cut at any level. 



While at Mr. Agassiz's Newport Laboratory during the sum- 

 mers of 1892 and 1893 and the U. S. Fish Commission Station at 

 Woods Holl, Mass., during April , 1893 , I tried the experiment indi- 

 cated above. 



Obelia commissuralis occurs abundantly upon the rock- weed at 

 low tide mark. The normal individual is composed of a stem and a 

 hydranth. The stem is composed of three parts, which I designate 

 the proximal segmented tract, the unsegmented tract, and the distal 

 segmented tract. Beyond the latter lies the hydranth. Making the 

 letter r stand for ring or segment, h for hydranth, and ut for 

 unsegmented tract, the normal formula of the individual of this 

 species may be given as 3r -\- ut + 3r + h. Variations in the 



in erster Linie von ihrer eigenen Natur, d. h. von der Zusammen- 

 setzung .ihr e s Idioplasmas. Die das Id. zusammensetzenden 

 „Determinanten" bestimmen, was weiter aus dieser Zelle und aus allen 

 ihren Nachkommen werden soll," 



