592 
ME. W. K. PAEKEE ON THE STEUCTHEE AND DEVELOPMENT 
they become large and complex ; and of the three principal pairs the foremost and hind- 
most form secondary graftings and intimate blendings with the axial skeleton. The 
middle pair, also the eyes, have the surrounding parts cunningly built over and around 
them ; sockets are sunk in the skull for their reception, albeit they are free themselves. 
A temporary change in the direction of the axial nervous mass in front, its meso- 
cephalic flexure , whereby the straight embryo is formed into a crozier-like body, this, 
of necessity, is modifying the growth of the axial skeleton as long as the head is thus 
bent. 
Now the beautiful researches of Mr. Balfour show that the notochord becomes (in 
the Selachians) shaped like a sheep-hook * during the period of embryonic growth ; and 
my own researches (“ On the Skull of the Shark and Skate,” Trans. Zool. Soc. vol. x. 
plate 35) show that the part of the mesoblastic plates just in front of this bend grow 
more rapidly than the part behind, from which they have been, as it were, dislocated. 
Time has now to be considered in the morphology of the skull ; and parts that start 
first and grow quickest generally overshadow the later and slow-growing parts. 
We thus get a morphological anachronism — some elements of the skeletal structure 
standing still and waiting, apparently suppressed, until the proper nick of time occurs 
in the age and growth of the animal. 
But for the modifications undergone by the cephalic structures, the vesicular condition 
of the neural axis, the development of the organs of special sense, &c., the two plates 
that run along the sides of the notochord might have been chondrified at one and the 
same time from end to end of the animal, a little slowness being allowed for the 
extreme ends. 
This would not have been materially affected by the somatomic subdivision of the 
tracts ; they might have been separated into moieties for each vertebra, or, obliterating 
the earlier divisions, chondrification may be imagined as running on (as it does in the 
neck of Selachians) along parallel tracts continuously. 
But in the existing Vertebrata chondrification does not take place in the basal meso- 
blastic tracts of the head at one and the same time ; and it would be well if we knew 
what causes this anachronism. 
This is extremely difficult to account for. It is not equally seen in all types; and 
there are remarkable variations within the limits of an Order, or even Family. 
On the whole, the trabeculae, or interocular tracts, chondrify first; then the inter- 
auditory, or parachordals ; and, lastly, the internasal, or fore ends of the trabeculae. 
The development of true (arrested) vertebrae in the head is possible as far as the 
three essential elements go, namely, the notochord and its pair of investing mesoblastic 
tracts. 
As a fact, such segments, even below the hind brain, are imperfect and transient; 
under the other two vesicles they are impossible, and in the early embryo only slight 
traces of somatomic division can be seen, even in the region of the hind brain. 
* Journ. of Anat. and Phys. vol. x. plate 24. 
