PRELIMINARY ACCOUNT OF DEVELOPMENT OF LAMPREYS, 149 
while it has become extremely small in the sexual animal, 
and is almost obliterated by the enormously swollen genital 
glands. 
The mouth is one of the most peculiar organs of the entire 
organism, and in its development, I believe, the key to many 
of the peculiarities of the Cyclostomata is to be found. It arises 
as a simple sinking in of the epiblast, which becomes gradually 
deeper until it finally touches the hypoblast at the blind anterior 
end of the alimentary canal. The next step is the perforation 
of the two membranes, which seems to take place in the usual 
way; but I have not been able to follow all the steps in the 
process. The great peculiarities of the mouth-parts which we 
have mentioned, lie in the lips, &c., which surround the cavity, 
and will be more properly treated of in connection with the 
mesoblast. 
The epidermis is at first single-layered, and does not divide 
itself into two layers until after the hatching of the larva. 
The central nervous system has, so far as the earlier stages are 
concerned, been carefully investigated by Calberla (‘ Morph. 
Jahrb.,’ Bd. iti), and I can confirm his results in all points, except 
that the division into two layers of the epiblastic cells concerned 
in the formation of the cerebro-spinal axis does not seem to be 
so clear as he made it out. A shallow groove appears in the 
dorsal surface of the embryo, and the epiblast cells in the neigh- 
bourhood of this groove begin to divide themselves rapidly into 
two layers, and by their multiplication form a strong solid keel 
which projects inwards towards the hypoblast. The keel is soon 
detached from the general epiblast, becomes oval in section, and 
upon the fourteenth day after hatching develops a lumen by the 
separation of some of the cells. Such a peculiar formation of the 
medullary cord is to be found only in the Teleosteans. Now, it 
has been suggested by other investigators that the Teleostean egg 
has undergone a reduction in bulk through the partial loss of 
the food material. If we assume such a reduction in bulk in 
the egg of Petromyzon, we shall be able to explain not only the 
surprising correspondence of two such widely separated groups, 
but also the presence of hypoblast cells in the roof of the seg- 
mentation cavity in the egg of Petromyzon, which is supplied 
with such a small amount of food material. 
The drain arises at first as a club-shaped swelling of the 
anterior end of the nervous axis, and is very small and simple. 
Soon, however, the rudiment becomes divided by shallow con- 
strictions into three divisions, of which the posterior is by far 
the longest, the anterior the shortest. The walls of the b ain 
are everywhere uniform ; thickenings and thinnings of separate 
parts do not occur till a much later period. ‘The rudiment of 
