482 PATTEN AND HAZEN. [Vol. XVI. 



During and after the Trilobite stage the lateral arm of the 

 duct becomes convoluted and a second loop is formed near its 

 proximal end, directed backwards and medianly, and lying dorsal 

 to the median arm of the first loop (cuts 3 A. and 3 B.). The 

 median arm of the first loop becomes considerably dilated, and 

 apparently acts as a reservoir for the secretions of the gland. 

 It remains a straight tube throughout life. The anterior end 

 of the lateral arm (/.«.) is smaller and somewhat convoluted, 

 the foldings increasing in number and extent towards the pos- 

 terior loop. 



The structure of the nephric duct during the Trilobite stage 

 is shown in PI. XXV, Fig. 45. The cells lining the duct now 

 have no distinct cell walls, although the walls are easily seen in 

 the preceding and in the following stages. 



The anterior loop of the duct now extends as far forward 

 as the second leg. The following parts may be distinguished 

 (cut 3 A.) ; viz. : {a) the short ectodermic portion ; {b) the dilated 

 median arm of the anterior loop ; {c) the slightly coiled lateral 

 arm of the anterior loop; {d) the much coiled posterior loop; 

 {e) the end funnel; (/) the end sac. 



B. The End Sac. — The early stages in the formation of 

 the end sac out of the fifth thoracic somite have already been 

 described. 



Before the Trilobite stage a longitudinal section shows the 

 presence of a few enlarged finely granular cells in the walls 

 of the end sac. These granules increase in size and numbers 

 till the cells present the appearance shown in PI. XXIV, Fig. 

 42, e.s. They now resemble those cells from which the nephrid- 

 ial tubules develop. 



In the Trilobite stage (PI. XXVIII, Fig. 80) the coarse gran- 

 ular protoplasm has nearly disappeared, and the sac is lined with 

 a delicate layer of protoplasm, with here and there a nucleus. 

 Numerous finger-like evaginations of the wall of the sac have 

 developed, the walls of which have the same structure as those 

 of the sac itself. 



It was not possible to determine the exact manner in which 

 these evaginations were formed, A careful study indicates that 

 the large tubes opening directly into the sac were formed as 



