DEVELOPMENT OF THE SKULL IX THE BATRACHIA. 17 



that the fore bram is turned downwards and somewhat backwards, and the mid brain 

 is tilted upwards and forwards, and forms the actual end of the embryo, lying against 

 what is called the " frontal wall. """' 



The fore and hind vesicles are thus brought into contiguity, and the organic apex, 

 or real fore end of the embryo, is a little in front of the hind vesicle ; it looks down- 

 wards and a little backwards. 



At that point a remarkable process takes place ; the notochord turns downwai'ds, 

 taking, as the skeletal axis, the same direction as the neural axis ; the fore brain after 

 a tune sends a budding jJi'ocess out, and the oral mucous membrane sends a budding 

 process in, and the two, gradually becoming mutually engrafted, the one upon the 

 other, open freely into each other, and then the lower process becoming closed below, 

 we have the pituitary body formed. 



In tliis quasi-archaic condition of the head and its parts, the bodi/ is a mere 

 appendage, solid hyaline cartilage forms first where most pressure is, and the paraxial 

 bands grow first of all from near the apex of the notochord to near the fronted wall. 



But the fore brain swells and hangs down ; the skeletal parts respond to this 

 condition and wind theii- way round its base, embracing its sides, and then meet, or 

 nearly so, in front, to diverge again in the nasal region of the face. 



After a few days the paired bands have developed backwards under the hind brain 

 (ibid., figs. 3 and 4), and behind the head they appear in patches that alternate with 

 the somatomes (muscle-segments, &c.) along the spinal region. 



Then, in Tadpoles Jive-ttrelfths of an inch long {op. cit., Plate 55, fig. 3), the 

 chondrocranium is simply a pair of planks on wliich the brain lies ; it has become 

 much straighter, but the mid brain lies high, still. 



Strength, however, has been gamed by fusion at two points (in the antorbital and 

 postorbital regions), and between these two points the bars are becoming crested ; 

 these ridges are the beginning of the ethmoidal and sphenoidal side walls. 



Keturning to the earhest stage we find that there are, indeed, two i)au-s of cartilage 

 on each side nearly equal, with a third pair of much shorter bars. 



In these temporary sucking fishes the cliief parts of the organism are all crowded into 

 the head and throat ; aU else is then- paddling fin, and the creature is called a bull- 

 head, on account of this cephalic preponderance. 



The second, more external, cartilaginous band {pd., q.), carrying at its end a short 

 inturned segment {Mk.), is the "pier" of the mandible, with its free swinging joint, 

 the inidinient of the lower jaw. 



Under that pier, one-fo\irth of the way from its distal end, the third cartilage is 

 seen {c.hy.) : this is the free jouit of the hyoid {lingual) arch whose pier or epi-hyal 

 element does not appear for three or four months to come, and when developed is 

 devoted to new purposes, does not become the practical " suspensorium " of the hngual 

 arch at all, but fonns jiart of the outworks of the auditory labyriath. 



• See Balfour's ' Elasmobranchs,' plate 7. 



MDCCCLXXXI. D 



