83 
and in the oldest embryo the last gill opening is opposite the twenty- 
ninth spinal ganglion. Thus it is plain that at one time the kidneys 
are present in the entire region where later the gills of the adult 
come to exist, and also in several segments anterior to this. The 
tubules lose their connection with the body cavity, and about 20 of 
the posterior ones entirely disappear. In the anterior region both duct 
and tubules disappear. The part of the system in the one or two 
segments just back of the gills is converted into the so-called head 
kidney of the adult. This takes place at a comparatively late stage, 
but the series was not sufficiently complete to allow the process to be 
followed. The remaining tubules, about 30 in number, acquire glo- 
meruli, and form the so-called mesonephric tubules of the adult. 
A comparison of the structure of the embryo of stage C with the 
structure of the adult proves that the development is direct, that is, 
without metamorphosis. 
I wish now to add to the above a few remarks on the eye, the 
ear, and the cranial nerves. 
In all stages the eye is very small in proportion to the size of 
the head. In stage A it is already an optic cup, the inner, or retinal 
layer of which is several cells thick, while the outer layer is one 
cell thick. In none of the embryos at my disposal have I been able 
to detect pigment in this outer layer. The optic stalk is hollow, but 
becomes solid at the point where it joins the brain. The hollow of 
the stalk is continuous with the space between the inner and outer 
layer of the optic cup. A choroidal fissure is present. In the youngest 
embryo there is a cone-shaped thickening of the external epiblast, the 
apex of which comes in contact with the anterior edge of the optic 
cup. This is no doubt homologous with the thickening of the epiblast 
which in other vertebrates gives rise to the lens. The fact that it does 
not lie directly in the mouth of the optic cup is to be accounted for 
by the fact that the eye is directed slightly backwards. Traces of 
this thickening can be seen in an other embryo of stage A. 
In stage B there are no essential changes, except that there are 
no traces whatever of the lens. In stage C the choroidal fissure has 
disappeared, and the optic stalk has been replaced by the optic nerve. 
This is a slender nerve, that can be traced distinctly from the eye 
to its connection with the brain. The eye is still an optic cup, and 
this is true also for the eye of the adult. The eye is evidently rudi- 
mentary, as is proved by the appearance, and subsequent disappear- 
ance of the lens thickening. 
The ear in stage A consists of two parts; auditory vesicle, and 
6* 
