8l6 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



does not become shut off from the body-cavity, and also bends 

 inwards towards the middle line. The inflected part of the duct 

 is the first rudiment of the pronephros, and very soon becomes 

 considerably dilated relatively to the posterior part of the duct. 



The posterior part of each segmental duct acquires an opening 

 into the cloacal section of the alimentary tract. Apart from 

 this change, the whole of the ducts, except their pronephric 

 sections, remain for a long time unaltered, and the next changes 

 we have to speak of concern the definite establishment of the 

 pronephros. 



The dilated incurved portion of each segmental duct soon 

 becomes convoluted, and by the time the embryo is about 10 

 millims. in length, but before the period of hatching, an important 

 change is effected in the relations of their peritoneal openings 1 . 



Instead of leading into the body-cavity, they open into an 

 isolated chamber on each side (Plate 38, fig. $i y pr.c.), which we 

 will call the pronephric chamber. The pronephric chamber is not, 

 however, so far as we can judge, completely isolated from the 

 body-cavity. We have not, it is true, detected with certainty at 

 this stage a communication between the two ; but in later stages, 

 in larvae of from 11 to 26 millims., we have found a richly ciliated 

 passage leading from the body-cavity into the pronephros on 

 each side (Plate 38, fig. 52, ///.). We have not succeeded in 

 determining with absolute certainty the exact relations between 

 this passage and the tube of the pronephros, but we are inclined 

 to believe that it opens directly into the pronephric chamber just 

 spoken of. 



As we hope to shew, this chamber soon becomes largely 

 filled by a vascular glomerulus. On the accomplishment of 

 these changes, the pronephros is essentially provided with all 

 the parts typically present in a segment of the mesonephros 

 (woodcut, fig. 4). There is a peritoneal tube (/) 2 , opening into 

 a vesicle (v) ; from near the neck of the peritoneal tube there 



1 The change is probably effected somewhat earlier than would appear from our 

 description, but our specimens were not sufficiently well preserved to enable us to 

 speak definitely as to the exact period. 



2 We feel fairly confident that there is only one pronephric opening on each side, 

 though we have no single series of sections sufficiently complete to demonstrate this 

 fact with absolute certainty. 



