334 THE ORIGIN OF VERTEBRATES 
region, where in the Ammoccetes only blood and fat-containing cells 
are present. For this purpose Shipley kindly allowed me to examine 
his series of sections through the embryo at various ages. These 
specimens are very instructive, especially those stained by osmic 
acid, which preserves the natural thickness of this space better than 
other staining methods. At an age when the branchial cartilages are 
seen to be formed, when no fat-cells are present, a distinctive tissue 
(Fig. 133, B) is plainly visible in the velum and at the base of the 
tentacles, in the very position where in the more advanced Ammo- 
ccetes muco-cartilage exists. Taking, then, this tissue as our guide, 
the specimens show that the space between the skin and the visceral 
muscles in which the cartilaginous basket-work lies is filled with a 
similar material. At this 
stage a sheet of embryonic 
tissue occupies the posi- 
tion where, later on, blood- 
spaces and fat-cells are 
found, and this tissue re- 
sembles that seen in the 
velum and other places 
where muco-cartilage is 
afterwards found. 
I conclude, therefore, 
Fig. 183.—A, Mvco-Carrinace oF VuELUM; ens 
B, Empryonic Mvuco-CartinaGE oF TENTACU- that originally the bran- 
LAR Bar. chial or mesosomatic re- 
gion was covered with a 
dorsal plate of muco-cartilage, which carried on its under surface the 
dorsal part of the branchial basket-work, and sprang from the central 
core of skeletogenous tissue around the notochord; this plate was 
separated from the plate which covered this region ventrally by the 
lateral grove in which the gill-slits are situated. The ventral plate 
carried on its under surface the ventral part of the branchial basket- 
work, and was originally continuous with the plate over the thyroid 
gland. 
In Fig. 134, A, B, ©, the cranial skeleton of Ammoccetes is 
represented from the dorsal, ventral, and lateral aspects. The 
muco-cartilage is coloured red, the branchial or soft cartilage blue, 
and the hard cartilage purple. The degenerated muco-cartilage of 
the branchial region is represented as an uncoloured plate, on 
