Structure and Development of the Nephridia. 395 



wall of thìs region is vacolated and contains numerous coarse deeply- 

 staining: grauules, whicb are also present in the adjoinÌDg- eetoderm; 

 but no sharply defined terminal vesicle is as vet present. In the 

 nephridium represented in Fig. 42 (2"'^ nephridium of a somewhat 

 more advanced larvai the conditions are essentially similar. 



In Piate 25, Fig. 45, a represeutatiou of a cross section of a stili 

 more advanced stage, the earliest condition of the vesicle is seen. 

 The terminal section of the nephridium has become dilated and has 

 acquired thiuner walls which however in ali other respects are similar 

 to those of the glandulär regions proper. The Strand of muscle- 

 fibres which extends over the nephridium at the junction of the thick- 

 walled glandulär region represents a portion of the originai longitu- 

 dinal muscle-layer (which has already become associated with the 

 terminal region at the stage of Fig. 40). These muscle-fibres con- 

 stitute the entire early musculature of the organ; later an extensive 

 network of muscle-fibres is formed over the enfire surface of the 

 vesicle. 



Figs. 52 — 55, Piate 25 represeut a later condition of the vesicle, 

 which has become much enlarged, and well defined with reference 

 to the adjoiniug glandulär region. The most striking characteristic 

 of the vesicle in this larva was the presence of the large intensely 

 staiuing granules, both in the wall of the vesicle and in the neighboring 

 eetoderm (see Fig. 51). The presence of granules in the eetoderm 

 of this region is a common phenomenon, and is somewhat difficult 

 to nnderstand. Possibly the granules in the preparation represent 

 drops of fluid excretory matters which find their way between the 

 ectodermal cells and collect there. No external opening at this 

 stage can as a mie be detected, and the contractions of the vesicle 

 may possibly force the excretory matters into the intercellular Spaces 

 of the eetoderm. 



Fig. 6a, Piate 22 represents the vesicle at a later stage in a 

 distended condition, showing the thin walls composed of tlattened 

 cells. In larvae of this stage the contractions of the vesicle may 

 readily be observed through the transparent body-walls. In later 

 development the number of epithelial cells increases greatly and 

 the cells themselves become columnar (see above p. 370 . The 

 musculature of the wall also undergoes a great extension. 



