MUSEUM OF COMPARATIVE ZOOLOGY. 227 



Figure 9 shows the fundament of the glomus in one of the older em- 

 bryos of this stage. Within the hollow of the fold may be seen two 

 cells (ms'cky.), which are to be regarded as the descendants of the first 

 small cells to which I referred in the younger embryo. Their differen- 

 tiation in the direction of connective tissue can be noticed throughout 

 the whole exteut of the fundament. The scattered rounded cells near 

 them probably represent embryonic blood cells in the region of the 

 aorta. 



Stage V. 



Plate I. Fig. 10. Plate IV. Figs. 31-34, 40. 



The embryos belonging to this stage are on an average about thirty 

 hours older than those of Stage IV. At this period almost all of the 

 eggs are hatched ; and, the duct having opened into the cloaca, the pro- 

 nephros becomes functional. The larvse of this stage measure 5-7 mm. 

 in length, the rapid increase in size being largely due to the growth of 

 the tail. 



The form which the pronephros presents in this stage has been studied 

 by means of reconstructions in the case of four pronephridia. The dia- 

 grams on Plate IV., Figures 31 to 38, represent in a rough way the num- 

 ber and distribution of the convolutions which the tubules present in this 

 and the following stage. Of these, Figures 31 to 34 relate to the present 

 stage. Figure 40 is a more accurate view of the pronephros which I 

 have diagrammatically represented in Figure 32. In Figure 40 the out- 

 lines were taken with but little modification from the original recon- 

 struction. I have not hesitated, however, even in this case, to remove 

 defects plainly due to artificial causes, such as distorted sections and 

 inaccurate superposition. 



Comparing this drawing with Figure 39, it is easy to follow the 

 changes that have taken place. In the earlier stage the fundaments 

 of the three tubules are already present. The first modification which 

 may be noted is the deepening of the constrictions which are indicated 

 between the successive nephrostomes. In this way are formed three 

 transverse tubules, joining distally a longitudinal canal ; the former are 

 the nephrostomal tubules, the latter I shall call the collecting trunk. 

 In this case the continuation of the collecting trunk pursues a nearly 

 straight course to the posterior margin of the gland, where it emerges as 

 the segmental duct. 1 A. second change which is apparent in Figure 10 



1 In Figure 40 the first nephrostomal tubule and the collecting trunk have a 

 pink color, the second tubule is yellow, and the third is orange, whereas the seg- 

 mental duct is uncolored. 



