Fig. 4.
Diagrammatic views of the pronephros of Lepidosteus.
A, pronephros supposed to be isolated and seen from the side; B, section through the vesicle of the pronephros and the ciliated peritoneal funnel leading into it; pr.n., coiled tube of pronephros; sd., segmental or archinephric duct; f., peritoneal funnel; v., vesicle of pronephros; bv., blood vessel of glomerulus; gl., glomerulus.
The different parts do not, however, appear to have the same morphological significance as those in the mesonephros.
Judging from the analogy of Teleostei, the embryonic structure of whose pronephros is strikingly similar to that of Lepidosteus, the two pronephric chambers into which the segmental ducts open are constricted off sections of the body-cavity.
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 pronephric 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 of 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, p.f.), and our failure to find them at this stage is not to be regarded as conclusive against their existence.