82 
The stages are designated as A, B and C. Of these, A and B 
are comparatively young, and are not widely separated from each 
other, while C is much older, and has many characteristics of the 
adult. 
The nose extends forwards under the brain, and in A ends blindly 
in two bilaterally symmetrical pockets; in B each pocket is divided 
into 3 or 4 secondary pockets. It is possible that a still younger stage 
would show an unpaired condition. There is a well developed hypo- 
physis, which extends backwards under the infundibulum, and in A 
comes in close contact with the alimentary canal at some distance 
from its anterior end, but there is no opening between the two. In 
B the nose and hypophysis have a common opening to the exterior, 
but in A no such opening exists. 
The mouth opening is formed very late; it is present in neither 
A nor B, although in B the conditions are such that one can say with 
almost absolute certainty, that, when it does appear, it will not be as 
a single median opening, but as two bilaterally symmetrical openings. 
In A a narrow canal extends from the anterior end of the mouth 
cavity into the cavity common to the nose and hypophysis. Its walls 
are probably of epiblast. In B the canal is closed. 
A great number of gill pouches appear in the course of de- 
velopment, perhaps as many as 35. Of these, the posterior 10—14 
develop into the gills of the adult, while the rest entirely disappear. 
Owing partly to its simplicity, and partly to the fact that it does 
not develop with equal rapidity in all portions of its length, it was 
possible to obtain a more complete account of the development of the 
kidney than of any other organ. It is possible, and indeed probable, 
that the system begins as a series of segmentally arranged thickenings 
of the somatic mesoblast, which are at first independent of one an 
other, at least there is one such independent thickening at the anterior 
end of the system in an embryo of stage A. One can say with cer- 
tainty that at an early stage the system consists of a series of seg- 
mentally arranged evaginations from the body cavity, which are connec- 
ted with one another by thickenings of the somatic mesoblast. 
Gradually these thickenings are converted into a ridge, and are 
then pinched off, and forma solid rod of cells. The lumina of the 
tubules extend into this rod, and it is thus changed into a duct, 
the segmental duct. The system extends through as many as 69 
Segments, and there are tubules in all of these except the last one 
or two. The entire system is evidently a pronephros. In one em- 
bryo at least, the system begins opposite the eleventh spinal ganglion, 
