1869.] 



the Skull o/Gallus domesticus. 



279 



tilaginous and the Osseous Fishes are never fairly out of sight. The Rep- 

 tile, and especially the Lizard, has been less helpful to me, on account of its 

 great 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 the "horns of the trabeculee " 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-nasal septum has been formed ; the " horns 

 of the trabeculee" have become the "nasal aloe," and an azygous bud of 

 cartilage has grown downwards between them; this is the "prsenasal" or 

 snout cartilage ; it is the axis of the intermaxillary region. At the com- 

 mencement 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 become struthious ; and now parosteal tracts (the an- 

 gular, surangular, dentary, &c.) appear round the mandibular rod. 



In this abstract I shall not trace the changes of the skull any further, 

 but conclude with a few remarks on the nomenclature of certain splints, 

 and as to the nature of the great basicranial bones. 



Some years ago I found that certain birds (for instance the Emeu) pos- 

 sessed an additional maxillary bone on each side ; knowing that the so- 

 called "turbinal" of the Lizard and Snake was one of the^maxillary series, 

 I set myself to find the homologies of these splints. Renaming the rep- 

 tilian bones " praevomers," on account of their relation to the vomer, and 

 supposing the feeble maxillaries of the Bird to represent them, I considered 

 that the true maxillaries were to be found in those newly found cheek- 

 bones of the Emeu and some other birds. 



After discussion with Professor Huxley I have determined to drop the 

 term " praevomer," and to call the supposed turbinal of the Lizard "septo- 

 maxillary," and the additional bone in the Bird's face <f postmaxillary." 



In many Birds, but not in the Fowl, the " septo-maxillary " is largely 

 represented — not, however, as a distinct osseous piece, but as an outgrowth 

 of the true maxillary. 



With regard to the basicranial bones, I have now satisfied myself that 

 the. " parasphenoid " of the Osseous Fish and the Batraehian reappears in 

 the Bird as three osseous centres — all true " parostoses," as in the single 

 piece of the lower types ; these three pieces are, the " rostrum " of the basi- 

 sphenoid and the two "basitemporals." 



These three centres rapidly coalesce to form one piece, the exact counter- 



