CLOSURE OF TUBULARIAN STEMS. 159 



pieces, there were none so large, unless the stem was thicker, or 

 a ridge intervened. 



Whatever the cause of closure may be, whether some kind of 

 contraction or not, the fact can clearly be seen from the study of 

 sections that cell-division has little or no part in the formation 

 of the closing membrane ; the new tube has practically the same 

 number of cells as the open, part-cylinder. Mitotic figures are 

 occasionally found, but not more than might have occurred in 

 a normal tubularian, and in any case they were not frequent 

 enough to indicate that mitosis is a factor in forming the closing 

 membrane. 



BRYN MAWR, 

 May, 1904. 



