296 PROCEEDINGS OF THE ACADEMY OF [May, 



In other words, it is a " regenerative ' ' as distinguished from a de- 

 generative process. Lowit (14) distinguishes between these two iu 

 the following: " Die erste fiihrt nacli nieine Auffassuug zurNeubil- 

 duug von Kern und Zelle, die letztere kann mit sekretorischen und 

 assiniilatorischeu Vorgangen ini Zellleibe zusainmenhangen und 

 durfte wahrscheinlich in vielen Fallen der ausdruck eines bevor- 

 stehenden Kern und Zelltodes sein," 



(2) Origin of the Ti/phlosole. — In specimens of Porcellio spin- 

 icornis 1.8-2 mm. long I have been able to follow the origin of the 

 typhlosole. This structure, as we have seen, is but an infolding of 

 the mid-dorsal wall of the " mid-gut," accompanied by a secondary 





Fig. 3. — Transverse sections of empty intestine of a young Porcellio 

 (1.8 mm.). X 180. A. At extreme anterior end of the "mid-gut"; B. 

 Sixth section (.04 mm.) posterior to A.; Typh.Ev. Primary evagination 

 in formation of the typhlosole ; Typh.lnv. Secondary invagination of 

 median two rows of cells which form the middle portion of the typhlo- 

 sole. 



outfolding on either side which appears as a ridge; but it is so 

 definite in form as to be })laiuly a specialization as well, and my 

 observations show that its development is a progressive one. 

 Cross sections of the anterior portion of the " mid-gut" in ani- 

 mals just hatched show a perfectly uniform imfolded wall, the 

 typhlosole not yet to be seen. In fig. 3, A, of a specimen 1.8 

 mm. long (taken in February and therefore probably several 

 months old), the section is from the extreme anterior end ; the next 

 (fig. 3, B) is only six sections (about .04 mm. ) farther back. The 

 latter shows the earlier stage. The mid -dorsal portion of the wall 

 is bulged out over a considerable extent of the circumference. The 

 cells composing the bulged iX)rtion are very open iu structure, like 



