Observations on the development of the excretory system in Turtles. 691 



vary iu width from ten to ninety niikra, more frequently from twenty 

 to forty mikra. They extend down towards * the inner ends of the 

 tubules and push out the endothelium of the coelom, forming bubble- 

 like lacunae around the tubule and chiefly about the opening of the 

 funnel into the coelom. Towards the end of the pronephric region it 

 is, at first, often difficult to tell whether the tubule has been cut off" from 

 the coelom or whether the opening is merely covered by the lacunae 

 which are distended (perhaps temporarily) from either side until they 

 meet (see Plate 47, Figs. 48 — 50). A careful study shows that even if 

 the lacunae of the glomus meet across the funnel , liquid from the 

 coelom could readily pass through their loosely built walls. Further- 

 more, at the tenth somite, the lining of the coelom, passing solidly 

 across this region, with close- set cubical cells, not only closes the 

 funnels but confines the lacunae of the glomus over the short distance 

 they still continue. Therefore, this last condition alone can be con- 

 sidered as the actual closing otf of the tubules (see Plate 47, Fig. 51). 

 Exceptionally in the ninth somite of embryo IV (1) there is one closed 

 funnel before the first open one and a second closed funnel between 

 the first and second open ones. 



In embryos of stage IV and older ones we see in the last somite 

 of the pronephric region the first appearance of the glomeruli. They 

 lie within the tubules and overlap for a short distance the glomus al- 

 ready described. This first appearance of the glomerulus is seen in 

 Embryo V (2), somite X, section 9 (Plate 47, Fig. 51). The description 

 will be found under the head of the mesonephros. 



Stages VII— XVI. 

 The specimens now prepared of stages VII and VIII are clearly 

 abnormal, and stages IX and X of Flatypeltis are wanting. We will 

 therefore next consider stages XI and XII. Here we find the following 

 differences in the pronephric region. The coelom does not extend so 

 far forward and we find the nephric elements at the extreme point 

 of the coelom, and even extending into the sclerotome anterior to it. 

 Embryo XII, 4, shows as the most anterior nephric element, the duct 

 beginning on the left at the fourth section of the seventh somite, im- 

 bedded in the sclerotome. In the eleventh section we come to the 

 end of the coelom, and here there is a rudiment of a tubule also. 

 At the twenty third section, a distinct nephric basis is seen on the 

 right side. The lacunae are more or less filled up with vascular 

 tissue, giving the glomus the appearance of vascular tufts, extending 

 into the coelom, or closed off by the endothelium at the tenth somite. 



