1902.] NATURAL SCIENCES OF PHILADELPHIA. 297 



those found in the extreme posterior end of the adult typhlosole. 

 They are larger in all dimensions than those in the rest of the wall, 

 so that the effect is such as would be produced by swelling. Since I 

 have found the structure identically the same in five different 

 individuals of the same age, killed in three different fluids, it seems 

 certain that the swelling is normal and due to some intrinsic growth 

 energy. Fig. 3, A, shows that the evagiuated or Indged portion 

 on the dorsal side is but preliminary to an invagination of the 

 median two cells whereby the mature form of the typhlosole is 

 approached. These cells have only to grow farther into the lumen 

 and to expand laterally, so as to cover or fill the inner grooves 

 formed at the sides, in order to attain the adult form. The invag- 

 ination may involve more than these two cells ; the stalks separate 

 at their bases later, as shown in fig. 4, A, so that the division 

 between these and the next cells must be purely arbitrary. 



In the anterior portion the cells rapidly recover from the swell- 

 ing which is so plain a feature at the first evaginatiou (figs. 3 and 

 4), while in the posterior portion they retain this loose structure 

 permanently (fig. 4). Here also the middle cells are not turned 



Fig. 4. — Transverse sections intestine of young Oniscus (4.4 mm) X 

 270 : A, througli iinterior portion of the typhlosole ; B, through posterior 

 end of the typhh)sole, same magnification ; Oh., channel for passage of 

 secretion or liquid food; Cg., coagulum of secretion of heiialopancreas. 

 B shows the elFects of killing tluids on tiie large cells of the typhlosole. 



in so far, while the lateral extension at their luminal border is 

 rather wider than it is farther anterior. The posterior portion of 

 which we have been speaking is the spatula-like enlargement men- 



