692 EMILY RAY GREGORY, 



Some cilia can be found in stage VII and from stage XI they are 

 always found. What "cursory examination I have made of them in- 

 dicates that each cell in and about the mouth of the funnel aud for 

 varying distances up the tubule (at times almost to the duct) has 

 one large strong cilia. They can still be seen in embryo XYI (1) 

 (Plate 50). 



Even the most anterior tubules at this stage show the cells 

 greatly increased in size, with clearer, lighter, more glandular ap- 

 pearance, and many of them containing dark staining granules. Many 

 blood corpuscles are now found in the glomus. The increased size of 

 the tubules and of the cells of necessity pushes out the endothelium, 

 so that, except at the very anterior end, we have a decided kidney 

 lobe. The development of the tubules and of the glomera may be 

 seen in the drawings of PlatypeUis embryo XII (3) [Plate 47, Figs. 52 — 

 56]. I have drawn somewhat more of the body than was necessary in 

 order to show the relative position of the parts at this stage. It is 

 somewhat difficult to follow the somites exactly by the time we have 

 reached this stage, and thus the later history of the pronephric region 

 is difficult to determine positively. It appears to me, however, that 

 the whole is pushed back and crowded into a small space, as the 

 head and neck develop, but that, nevertheless, the front end is over- 

 taken by the sclerotome, and cut off from the coelom. This gradually 

 degenerates though the arrangement of the cells may remain for a 

 considerable time. 



In a somewhat older embryo we find that all the tubules still 

 showing an opening into the body cavity (although very small) lie 

 within three somites. In stage XVI, what remains of the pro- 

 nephric region may readily been seen in the sagittal drawing, Plate 50, 

 which shows the pronephric glomera still hanging free in the body cavity 

 at the anterior end of the mesonephric lobe. Other sections of the 

 same series show four tubules opening into the coelom at this place. 



Briefly, the pronephros is composed of tubules, arising segmentally 

 from the somatic layer of somites 4—10, but early becoming more 

 numerous, which on the median side open into the coelom and out- 

 wards either end blindly or open into the duct formed by the fusion 

 of their ends. The funnels and lower parts of the tubules are sur- 

 rounded by lacunae furnished with blood from the aorta, which may 

 anastomose from point to point, and which in later stages acquire 

 some tissue and push out into the body cavity as external 

 glomera. 



