DEVELOPMENT OF THE SKULL IN THE LACERTILIA. 
619 
The ear-capsule (an.) is a short-necked flask, embedded in the subdorsal region of 
the head, opposite the hind part of the hind brain (C 3 ). 
The organs of support are, as yet, young cells : the mother-cells of the tissues that 
harden into hyaline cartilage, into the fibrous webs, or into bone. 
Yet how important the changes are that the embryo has already undergone it is 
easy to see, for everything has been marked out; and proliferating cells, in countless 
numbers, are ready to breed, and indeed are breeding the filial cells that will be 
transformed into the various tissues and organs. 
Second Stage. Embryos of Lacerta agilis, 3 lines inch) long. 
In this larger embryo the “ somatomes ” have increased from thirty to fifty, and the 
tail is now curled upon itself. 
The allantois (all.) is now applying itself, in a discoid form, to the inside of the 
chorion, but the abdominal walls are still very imperfect, and the pericardium (peel.) 
lies exposed on the concave ventral aspect of the embryo. 
To all appearance, the head in this stage is like the last, only larger ; but the fifth 
post-oral fold (hr.) is now distinct, and the neck of the ear-sac is more evident. 
The pituitary body is, at present, a mere fold of the oral lining; but the “ pleuro- 
peritonal,” or body cavity, is now represented by a separate cavity in each of the 
visceral arches, from the palatine to the fifth post-oral or the third branchial. 
I shall describe these structures in the next stage, in embryos one-fourth larger than 
this, for in them the metamorphosis of the parts is taking place rapidly, which makes 
them very instructive. 
Third Stage. Embryos of Lacerta agilis, from 4 to 5 lines long. 
A good supply of embryos at this stage has enabled me to dig about the roots of my 
subject more than I am wont, yet in this it is necessary to exercise self-denial; if I 
did not, the special morphology of the skull would be lost in general embryological 
details. 
At this stage, in which the embryo is budding and swelling with life, the differen¬ 
tiated groups of cells taking root downwards, and bearing fruit upwards—organs and 
“ elements ” of all sorts growing as fast as gourds, and yet with a perfectness of working 
that is as amazing as its rapidity-—the eye and the mind of the close observer are 
kept in a continual dazzle, and it is no easy task to watch this, and to attend to that. 
Here, if anywhere, I am grateful for the co-operation of such workers as Balfour 
and Marshall ; and here, especially, does our work overlap and dove-tail. 
Besides the development of the brain, cranial nerves, and organs of special sense, I 
must refer also to Mr. Balfour’s account of the development of the “ pituitary body 
the “visceral clefts” and segmentation of the head; and the “ body-cavities” and 
“myotomes” of the head (‘Elasmobranchs,’ plate 14, pp. 189, 206, and 211). 
