417 
STRUCTURE AND DEVELOPMENT? OF LEPIDOSTEUS, 
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 import¬ 
ance which we have noticed is the formation of a vascular projection into the prone- 
phric 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 25, 
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 25, fig. 53, p.f.), 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 
formation of segmental tubes to fill up the space between the pronephros and 
mesonephros. 
The two segmental ducts have united behind into an unpaired structure in an 
embryo of 11 millims. This structure is no doubt the future unpaired urinogenital 
chamber (Plate 26, figs. 58 A, and 60, bl). Somewhat later, the hypoblastic cloaca 
becomes split into two sections, the hinder one receiving the coalesced segmental 
ducts, and the anterior remaining continuous with the alimentary tract. The opening 
of the hinder one forms the urinogenital opening, and that of the anterior the anus. 
In an older larva of about 5'5 centims. the pronephros did not exhibit any marked 
signs of atrophy, though the duct between it and the mesonephros was somewhat 
reduced and surrounded by the trabecular tissue spoken of in connexion with the 
adult. In the region between the pronephros and the front end of the fully developed 
part of the mesonephros very rudimentary tubules had become established. 
The latest stage of the excretory system which we have studied is in a young Fish 
of about 11 centims. in length. The special interest of this stage depends upon the 
