92 
PROGRESS OF MICROSCOPICAL SCIENCE. 
obscuration, the mouth of the tadpole is strangely modified in harmony 
with its “ suctorial ” character and affinities (showing a remarkable 
affinity to the mouth of a lamprey), so that a whole system of carti- 
lages has to be eliminated from the lips before the mouth ( proper) can 
be understood. The labial system is slightly and slowly developed in 
the salmon, and its mouth is thus much more in harmony with that of 
the embryo reptile or bird than with that of the tadpole. After the 
simple stage is passed, the development of the facial arches is very 
different in the two types, as different, indeed, as in any two possible 
examples that could be given in the whole vertebrate group. The 
facial arches behind the mouth now undergo segmentation ; first the 
hyoid, and then the mandibular. The hyoid is cloven from top to 
bottom, and also has a single distal piece separated off. “At this stage 
we get an explanation of what is seen in certain rays, where the hyoid 
suspensorium is permanently double ; and also ascertain that this 
second postoral arch, which retains the anterior piece in relation to 
the skull as the great ‘ hyomandibular’ pier, does not need the saw of 
the transcendentalist to put it into proper relation to its surroundings. 
Nature’s invisible wedge has done what was needed, and the supposed 
double rib turns out to be half a visceral arch. On the whole, this 
second stage is extremely ‘ Plagiostomous,’ for the details of which I 
must refer to the main paper.” While in the egg the head of the 
embryo is flattened, and so twisted that one of the eyes (it may be the 
left or the right) looks upwards towards the “ chorion,” the other 
having a visceral direction. The facial bars, at first having all a 
simple sigmoid form, rapidly change towards the time of hatching, 
and, when the head gets free, the cerebral vesicles speedily swell, 
taking on the form so familiar to the embryologist; and the head now 
gains the “ mesocephalic flexure.” “ After this an approach is made 
to the Teleostean type of structure; but this is not done at a stride. 
The intermediate condition is thoroughly { Ganoid,’ and, happily, 
comes in to explain the related structures of the older and newer 
‘ Orders.’ I am not aware that any stage of the heart or of the in- 
testines shows either the many valves of the ‘aortic bulb’ or the 
intestinal spiral valve ; this must be seen to ; yet if these never show 
themselves in the ‘fry’ of the osseous fish, their absence does not 
affect the general skeletal morphology. The salmon amongst fishes, 
like the fowl amongst birds, never attains to the greatest degree of special 
class-modification ; it remains subtypical, with a dentigerous maxillary, 
a ductus pneumaticus, a very chondrosteus state of the skull, and a very 
heterocercal tail. Yet, from an ichthyological point of view, this fish 
is an immense height above the Sharks and Rays, and is far in advance 
as a fish of the whole group of ‘ Ganoids.’ The results of the grada- 
tional study cf the fish-forms by the zoologist, and of their secular 
study by the paleontologist, are both in harmony with morphological 
facts. Although the light obtained is but as the first streak of dawn, 
yet it is a pleasant light, and quite sufficient to show each kind of 
worker where and how to renew his owti special toil. I cannot close 
this brief abstract without remarking that my researches in these, the 
highest types of animals, seem to me to be in perfect accordance with 
