278 Mr. W.K.Parker on the Structure and Development of [Feb. 11, 



That paper was given as the first of a proposed series, the subsequent 

 communications to be more special (treating of one species at a time) and 

 carrying the study of the development of the cranium and face to much 

 earlier stages than was practicable in the case of the struthious birds. 



Several years ago Professor Huxley strongly advised me to concentrate 

 my attention for some considerable time on the morphology of the skull of 

 the Common Fowl ; that excellent advice was at length taken, and the paper 

 now offered is the result. 



A full examination of the earlier conditions of the chick's skull has cost 

 me much anxious labour ; but my supply of embryonic birds (through the 

 kindness of friends) * was very copious, and in time the structure of the 

 early conditions of the skull became manifest to me. 



The earliest modifications undergone by the embryonic head are not 

 given in this paper : they are already well known to embryologists ; and 

 my purpose is not to describe the general development of the embryo, but 

 merely the skeletal parts of the head, 



These parts are fairly differentiated from the other tissues on the fourth 

 day of incubation, when the head of the chick is a quarter 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 consoli- 

 dated 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 although 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 Tina- 

 mou, a bird which retains so many of its cranial sutures. 



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

 rately 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 Shark, 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 Car- 



* Dr. Murie is especially to be thanked for his most painstaking kindness in this 

 respect. 



