1872.] Mr. W. K. Parker on the Skull of Sal mo salar. 



361 



in nowise near akin to each other. In the very first stage, where there is an 

 essential agreement, in one important particular they greatly disagree ; for 

 the embryo of the salmon has two arches in front of its mouth, while the 

 tadpole has but one ; there is also an additional gill-arch in the osseous 

 fish. 



In the earliest stage of the salmon worked out by me I found a much 

 more distinct condition of the parts than in frogs at the same stage ; the 

 differentiation of the latter is obscure as compared with the fish, and this 

 not merely because of the quantity of pigmentum nigrum in the tissues of 

 the former. 



Then, in addition to other causes of obscuration, the mouth of the tad- 

 pole 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 cartilages 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 intestines shows either the many valves of the "aortic bulb" or 

 the intestinal spiral valve : this must be seen to j yet if these never show 



