1 86 WILSON: [Vol. I. 



of longitudial muscles, developed out of mesoblastic cells that 

 pass down from the overlying mesoblastic band. The nephridia 

 arise as paired metameric outgrowths from the ncpJiridial rows, 

 there being a single pair in each somite just behind and in con- 

 tact with the rudiment of the dissepiment. Each nephridium 

 appears to consist at first of a single cell, though it is difficult 

 to determine this with certainty. However this may be, it soon 

 assumes the form of a U-shaped cord of cells, that projects 

 into the coelom and becomes invested with flattened mesoblastic 

 cells. One limb of the U remains attached to the body-wall, 

 and ultimately forms the distal part of the nephridium ; the 

 other limb becomes free from the body-wall, but remains 

 attached to the dissepiment. Opposite to this point of attach- 

 ment, on the anterior face of the dissepiment, is a single large 

 mesoblastic cell, which gives rise to the ciliated funnel. The 

 subsequent development of the nephridium consists in, (i) the 

 elongation and convolution of the loop; (2) the perforation of 

 the central cord of cells from end to end, by a delicate canal, 

 and (3) the conversion of the mesoblastic covering into tne 

 outer layers of the organ. 



IV. The outer row of Cells. 



I am still in doubt as to the fate of the outer cell-row (r^, 

 Figs. I, 2). The teloblast from which it arises is precisely like 

 the nephroblast, and the cell-fow itself differs from the nephrid- 

 ial row only in being, as a rule, somewhat narrower. Passing 

 forwards, the row becomes less distinct, and is for a time very 

 hard to follow, whether in surface-views or in sections. In 

 early stages it sometimes fuses in front with the nephridial row, 

 but in some specimens it seems to be distinct. In later stages it 

 appears to be continuous with the band of cells {^x. Fig. 6) 

 described on p. 114. In embryos nearly ready to hatch this 

 band can no longer be distinguished, and I have been unable to 

 establish any connection between it and any adult structure. 



V. The Mesoblast. 



The mesoblastic bands arise as single rows of cells at the 

 latero-posterior angle of the mesoblasts, curve around their 

 outer sides so as nearly to meet in the median line, then bend 



