8l8 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



With the formation of the convoluted duct opening into the 

 isolated section of the body-cavity we may speak of a definite 

 pronephros as having become established. The pronephros is 

 placed, as can be made out in later stages, on the level of the 

 opening of the air-bladder into the throat. 



The pronephros increases in size, so far as could be determined, 

 by the further convolution of the duct of which it is mainly 

 formed ; and the next change of importance which we have 

 noticed is the formation of a vascular projection into the pro- 

 nephric chamber, forming the glomerulus already spoken of 

 (vide woodcut, fig. 4,gl.), which is similar to that of the pronephros 

 of Teleostei. We first detected these glomeruli in an embryo of 

 about 15 millims., some days after hatching (Plate 38, fig. 52, gl.}, 

 but it is quite possible that they may be formed considerably 

 earlier. 



In the same embryo in which the glomeruli were found we 

 also detected for the first time a mesonephros consisting of a 

 series of isolated segmental or nephridial tubes, placed posteriorly 

 to the pronephros along the dorsal wall ot' the abdomen. 



These were so far advanced at this stage that we are not in a 

 position to give any account of their mode of origin. They are, 

 however, formed independently of the segmental ducts, and in 

 the establishment of the junction between the two structures, 

 there is no outgrowth from the segmental duct to meet the 

 segmental tubes. We could not at this stage find peritoneal 

 funnels of the segmental tubes, though we have met with them 

 at a later stage (Plate 38, fig. 53, //.), and our failure to find 

 them at this stage is not to be regarded as conclusive against 

 their existence. 



A very considerable space exists between the pronephros 

 and the foremost segmental tube of the mesonephros. The 

 anterior mesonephric tubes are, moreover, formed earlier than 

 the posterior. 



In the course of further development, the mesonephric tubules 

 increase in size, so that there ceases to be an interval between 

 them, the mesonephros thus becoming a continuous gland. In 

 an embryo of 26 millims. there was no indication of the forma- 

 tion of segmental tubes to fill up the space between the pronephros 

 and mesonephros. 



