Structure aud Development of the Nephridia. 385 



of muscle-fibres in its interior. Fìg. 23, Piate 23 shows a 

 similar condition. The nephridium is tlius differentiated out of a 

 portion of the original mesoblast whieh in its early stages })ossesses 

 the sanie properties and undergoes the same transforniations as the 

 other portions of the lavar. 



At a slig-htlv later stage the nephridial eells become more readily 

 distinguishable from the adjoiniug purely myoblast cells. The lumen 

 beeomes more distinct and the body of the differentiating organ in- 

 creases in size, apparently in part at the expense of the protoplasm 

 of the adjoiniug mesoblast cells, which become visibly smallar. In 

 cross section the nephridial rudiment is soon observed to be more 

 prominent than the adjoining myoblastic cells, and its boundaries 

 appear better defined (Piate 23, Figs. 23 left, 19 left). Its proto- 

 plasm beeomes vacuolated and minute granules in many cases begin 

 to make their appearance in its interior ; the lumen beeomes pro- 

 gressively wider and better defined and the cilia become visible. 

 In the section represented in Fig. 19 (3 somites from the growing 

 zone) the nephridium of the left side is somewhat more advanced 

 than that of the right and is becoming sharply defined from the 

 adjoiniug myoblast cells, which bave perceptibly diminished in volume. 

 In the next somite of the same larva (Fig. 18) the boundaries of the 

 nephridium are quite definite, and flattened mesoblast cells are be- 

 coming applied to its surface to form the outer peritoneal covering 

 of the organ. The nephridium in the somite next in advance 

 (Fig. 17) is stili more highly differentiated; the lumen is more 

 spacious and the excretory granules are more uumerous. At this 

 stage the organ has the form of a simple tubule with an intracellular 

 lumen openiug anteriorly through the septum into the cavity of the 

 adjoiniug somite. The progressive histological diÖerentiation of 

 the early nephridia is well shown in the series of sections repre- 

 sented in these drawings (Piate 23, Figs. 16—21). 



As the nephridium advances in development the myoblast cells 

 gradually diminish in volume, while at the same timo the muscle-fibrils 

 in their interior increase in size (Figs. 16 — 21). Eventually the un- 

 ditlerentiated protoplasm or sarco})lasm is reduced to a thin layer 

 occupyiug the interstices between the fibrils aud contaiuing bere and 

 there a flattened nucleus^ In later stages (Piate 23, Figs. 25—28) 

 the muscle fibres become niuch more uumerous and more closely set 

 together, aud the sarcoplasm is reduced to au iucouspicuous inter- 

 fibrillar matrix in which the fibres are imbedded. The innermost layer 



