Zool— VOL. I.] TORREY—MONOGENESIS IN' METRIDIUM. 35 I 



The rate of fission is exceedingly slow, for I have not 

 observed a single instance of full severence of individuals, 

 though a number of dividing polyps have been kept in the 

 laboratory for nine months. The artificial conditions of 

 the aquaria may serve as a retarding factor. 



Some of the divisions are of marked inequality. In one 

 case three perfect pairs (i. e., in which both mesenteries 

 reach the oesophagus) and two half-pairs of complete mes- 

 enteries radiate from one oesophagus, nine perfect and three 

 half-pairs from the other; total, fifteen pairs. In fig. 3c five 

 complete mesenteries reach one oesophagus, ten the other; 

 total, eight pairs. Mesentery i, incomplete in fig. 3a and 

 3#, finally reaches the oesophagus in fig. 3c. In fig. 8 the 

 siphonoglyphs are unequally apportioned: the oesophagus 

 with two siphonoglyphs possesses two and one-half pairs of 

 non-directives, the monoglyphic oesophagus, five and one- 

 half pairs of non-directives; total number of complete mes- 

 enteries, eleven pairs. In fig. 4, fifteen mesenteries reach 

 the diglyphic, thirty-one the monoglyphic oesophagus; total, 

 twenty-one perfect pairs, four unpaired, or imperfect pairs. 

 Figures 5 (thirteen pairs) and 6 (eleven pairs) exhibit strik- 

 ing inequalities in apportionment of mesenteries between 

 the two oesophagi. In fig. 7, six of a total of eleven pairs 

 reach one oesophagus, five the other. In another case, 

 eleven and one-half pairs are distributed as equally as possi- 

 ble to the two oesophagi. So great are the discrepancies in 

 number and distribution of mesenteries in these cases, that 

 no definite relation between monogenesis and the number 

 and position of mesenteries can be asserted. 



Advanced stages of unequal fission may be distinguished 

 from budding usually by an external examination only, if 

 the polyp be living. The bases of the mesenteries are 

 readily seen as dark lines through the semitransparent 

 walls. In fig. 9 is represented a plan of the mesenteries in 

 the angle between the two individuals of a dividing polyp, 

 seen from above. Only two pairs of mesenteries from A 

 run into pairs from B. These form two T's, the stalks of 

 which run down the sides of the common column. All the 



