68 Royal Society : — 



of an inch (3 lines) in length ; this in my paper is termed the "first 

 stage." The next stage is that of the chick with a head from 4 to 

 5 lines in length, the third 8 to 9 lines, and so on. The ripe 

 chick characterizes the "fifth stage ;" and then I have worked out 

 the skull of the chicken when three weeks, two months, three 

 months, and from six to nine months old, the skull of the aged 

 Fowl forming the " last stage." 



During all this time (from their first appearance to their highly 

 consolidated condition in old age) the skeletal parts are undergoing 

 continual change, obliteration of almost all traces of the composite 

 condition of the early skull being the result — except where there is 

 a hinge, for there the parts retain perfect mobility. 



Here it may be remarked that althougli the Fowl is only an 

 approach to what may be called a typical Bird, yet its skull presents 

 a much greater degree of coalescence of primary centres than might 

 have been expected from a type which is removed so few steps from 

 the semistruthious Tinamou, a bird which retains so many of its 

 cranial sutures. 



The multiplicity of parts in the Bird's skull at certain stages very 

 accurately represents what is persistent in the Fish, in the Reptile, 

 and to some degree in certain Mammals ; but the skull at first is as 

 simple as that of a Lamprey or a Sliark, and, in the Bird above all 

 other Vertebrates, reverts in adult age to its primordial simplicity 

 — all, or nearly all, its metamorphic changes having vanished and 

 left no trace behind them. 



Although in this memoir I have no business with the Fish, yet all 

 along I have worked at the Fish equally with the Bird, the lower 

 type being taken as a guide through the intricacies of the higher ; 

 and here the Cartilaginous and the Osseous Fishes are never fairly 

 out of sight. The Reptile, and especially the Lizard, has been less 

 helpful to me, on account of its gi'eat specialization. 



On the fourth day of incubation the cranial part of the notochord 

 is two-thirds the length of the primordial skull, but it does not quite 

 reach the pituitary body ; it lies therefore entirely in the occipito-otic 

 region. The fore part of the skull-base extends horizontally very 

 little in front of the pituitary space ; this arises from the fact that 

 the " mesocephalic flexure " has turned tlie " horns of the trabeculse" 

 under the head. Thus at this stage the nasal, oral, and postoral 

 clefts are all seen on the under surface of the head and neck of the 

 chick. At this time the facial arches have begun to chondrify ; but 

 only the quadrate, the Meckelian rod, and the lower thyro-hyal are 

 really cartilaginous ; the other parts are merely tracts of thickened 

 blastema or indifferent tissue. 



In the second stage an orbito-uasal septum has been formed ; the 

 "horns of the trabeculse" have become the " nasal alse," and an 

 azygous bud of cartilage has grown downwards between them ; this 

 is the "pra^nasal" or snout cartilage; it is the axis of the inter- 

 maxillary region. At the commencement of this second stage the 

 primordial skull stands on the same morphological level as that of 

 the ripe embryo of the Sea-turtle ; at the end of this stage it has 



