184 DONALD WALTON DAVIS 



Recalling the types of division described above (p. 174 ff.)» it is 

 now clear how specimens with various numbers of siphonoglyphs 

 may arise. Diglyphic specimens most frequently give rise to 

 diglyphic forms, but may produce triglyphic (nos. 16b, 21a, 

 21b) or monoglyphic (nos. 16a, 21c) ones. Triglyphic indi\dduals 

 may produce diglyphic (no. 22a), monoglyphic, triglj^hic, or 

 tetraglyphic specimens. A tetraglyphic form (no. 15) did give 

 rise to one individual with four siphonoglyphs, and one with 

 two. Other possibiUties are at once evident. It is wholly prob- 

 able that any form may give rise, by one or two divisions, to any 

 form. Since a new siphonoglyph is invariably formed in the 

 new region at an early stage of regeneration, it is obviously more 

 appropriate to refer specimens in process of regeneration to their 

 ultimate class rather than to the one to which they appear at 

 the moment to belong. It is therefore no longer admissible to 

 say, as Davenport ('03, p. 141, line 20) has heretofore done, 

 referring to the usual type of division of a diglyphic individual, 

 'that "a monoglyphic S. luciae is the result of longitudinal division 

 of a diglyphic form," or to refer to the '^ metamorphosis of a 

 monoglyphic to a diglyphic type" (ibid., line 33). The true 

 monoglyphic type is a fixed type which does not develop a new 

 siphonoglyph, thus transforming into a diglyphic type, although 

 it may produce one or even two diglyphic individuals by a single 

 division. The division of a diglyphic individual in the great 

 majority of cases results in two diglyphic forms, never two 

 truly monoglyphic ones. 



Development of new mesenteries 



By the time the siphonoglyph is clearly differentiated, all of 

 the complete mesenteries to be formed may have appeared, but 

 they do not reach the esophagus until somewhat later. New 

 mesenteries show first in the lower half of the column. The 

 earliest evidence of a mesentery is a very thin sheet of mesogloea 

 extending from the mesogloea of the column wall inward into 

 the endoderm. When the sheet has pushed nearly through the 

 endoderm, the latter becomes projected inward in advance of it. 



