AND DEVELOPMENT OF THE BIRD'S SKULL. 101 



morphology after that of the Powl has been exhausted, so that the student's conception 

 of the Bird's skull may be richer and more in accordance with the fulness of Nature. 



I shall take the present opportunity to give figures of skulls that contain elements 

 frequently spoken of in other papers by me, but not illustrated, such as the " inter- 

 palatines " and " mesopterygoids." I shall also have to add other pieces, with their 

 names, namely, the " medio-palatine " and the " palato-maxillary." 



Processes that grow in a secondary manner out of the simple facial bars, for union of 

 bar with bar, serially, are called here " conjugational processes ;" the union of the right 

 and left bars of one arch at the mid line is called a " commissure." 



Further Researches on the Chick's Skull. 



My first stage (Phil. Trans. 1869, plate lxxxi. figs. 1 & 2) is rather underdated as the 

 fourth day ; those given are of the fifth, and those soon to be described run nearly to the 

 sixth day. The second stage (figs. 3-13) should be put as dating from the sixth and 

 seventh days of incubation. 



One of the first points to be determined in the morphology of the skull is the structure 

 of the " investing mass" of the notochord and the " rafters of the cranium" (" tra- 

 becular cranii"), the first pair of facial bars. 



The figures given in my former paper show the distinctness of these parts much more 

 definitely than I supposed at the time. These I now proceed to explain. At page 758 

 I speak of the ends of the investing mass as forming the cartilaginous " lingular sphenoi- 

 dales," thus confusing the free truncated end of the investing mass (plate lxxxi. 

 fig. 2, l.g), which looks forwards and outwards, with the free tops of the trabecular bars, 

 which are better seen in fig. 8, and in plate lxxxii. figs. 1 & 3, l.g. 



The explanation of these parts was made by the exquisitely lucid structures of the 

 embryo Salmon ; and now, if these figures of the Fowl be compared with those, it will be 

 seen that the trabecular apices are at first embraced by the squared out-turned ends of 

 the investing mass, and that in the next and third stages the curved trabecular tips, 

 where they surround the pituitary body, open out like the blades of a pair of callipers, 

 and growing more distinct and solid, form, in the third stage (plate lxxxii. figs. 1, 3, & 4, 

 l.g), the free tongue-like cartilages, the meaning of which I for a long while failed 

 to see. 



The first figure in the present paper (Plate XX. fig. 1, tr,pn) clears up another very 

 different point, namely, the formation of the " prenasal rostrum," or azygous con- 

 tinuation of the distal part of the trabecular arch. 



In my first paper, on the " Skull in the Ostrich tribe" (Phil. Trans. 1866, plate vii. p.n, 

 p. 122), the prenasal cartilage is described as being formed by confluence of the " tra- 

 becular ;" but in the second paper (p. 759) I have corrected this statement, but not without 

 falling into another error, namely, that of supposing the " alse nasi" to be formed directly 

 out of the " cornua trabecular." The anomalous position of the parts is caused by the 

 " mesocephalic flexure," the trabecular being positively hooked backwards. 



The alinasal, aliseptal, and the aliethmoidal cartilages are regions of membrane 

 which chondrify later than, but not separately from, the trabecule, the common 



p2 



