STRUCTURE AND DEVELOPMENT OE LEPIDOSTEUS. 
415 
the genital pore to form an unpaired vesicle. Their posterior parts are considerably 
enlarged, forming what Hyrtl calls the horns of the urinary bladder. In these 
enlarged portions, and in the wall of the unpaired urinary bladder, numerous trans¬ 
verse partitions are present, as correctly described by Hyrtl, which are similar to 
those in the female, but more numerous. They give rise to a series of pits, at the 
blind ends of which are placed the openings of the kidney tubules. The kidney duct 
without doubt serves as vas deferens, and we have found in it masses of yellowish 
colour similar to the substance in the vasa efferentia identified by us as remains of 
spermatozoa. 
II.— Development. 
In the general account of the development we have already called attention to the 
earliest stages of the excretory system. 
We may remind the reader that the first part of the system to be formed is the 
segmental or archinephric duct (Plate 23, figs. 28 and 29, sg.). This duct arises, as 
m Teleostei and Amphibia, by the constriction of a hollow ridge of the somatic meso- 
blast into a canal, wdiich is placed in contiguity with the epiblast, along the line of 
junction between the mesoblastic somites and the lateral plates of mesoblast. Ante¬ 
riorly the duct 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 hatch¬ 
ing, an important change is effected in the relations of their peritoneal openings.* 
Instead of leading into the body-cavity, they open into an isolated chamber on each 
side (Plate 25, fig. 51, 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 pro¬ 
nephros on each side (Plate 25, fig. 52 ,p.f.p.). 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. 
Th.e 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. 
MDCCCLXXXIJ. 3 H 
