Observations on the development of the excretory system in Turtles. ß97 



dorsal wall of the last tubules of the tenth somite, but similar cells 

 iu smaller groups may be seen projecting into the coelom, or irregu- 

 larly placed about the end of the tubule. Beyond the tenth somite 

 the glomerulus always arises at this point in primary tubules. In 

 stage XII the wall of the end of the tubule where the glomerulus is 

 found is formed by a single layer of flat, thread-like cells and is 

 sometimes difficult to find. The cellular nature of the external glomera 

 of the late stages of the pronephros is probably due to the fusion 

 of these irregular advance cells of the glomerulus with the lacunae of 

 the glomera. 



At first there is only a single glomerulus on either side, but at 

 stage XVI we find three, one above the other, and they undoubtedly 

 continue to increase with the continued development of the tubules. 

 They do not, however, develop so fast as the tubules; for in em- 

 bryo XII (3) it will be noted that the glomeruli dwindle and cease 

 while the tubules are still large. At this time and later their struc- 

 ture is slightly irregular, as may be seen from Fig. 69, Plate 49. 



The earlier branches from the aorta to the lacunae stopped at 

 the tenth somite, and it is not until some time later that the aortic 

 branches to the glomeruli appear. Up to stage XVI they are very 

 small and it sometimes seems almost as if the blood corpuscles push 

 their way through the tissues from the aorta to the glomeruli. From 

 this point on, one always finds many corpuscles in the glomeruli, so 

 that at times the tissue can scarcely be seen at all. 



The essential characteristics of glomus and glomerulus respectively 

 may be classified as follows: 



Glomus. Glomerulus. 



1. Origin : lacunar evaginations from 1. Proliferation of cells from wall 



the aorta. of tubule. 



2. Extent : 6th — 10th somites. 2. lOth somite to near cloaca. 



3. Position outside and around 3. Within the end of the tubule. 



tubules. 



4. Lacunae may anastomose and 4. Glomeruli remain distinct. 



glomera become more or less 

 united. 



5. In late stages becomes external. 5. Never external. 



The mesonephros is composed of non-segmental tubules arising 

 from the middle plate from about the 2nd somite to near the cloaca, 

 which in the pronephric region generally fuse with pronephric tubules, 

 and open into the coelom at the median end and the duct at the 



