33- 



4 



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 

 . T.^X j stage a sheet of embryonic 

 ~"A 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, 

 that originally the bran- 

 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, C, 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 



A B 



Fig. 133. — A, Muco-Cartilage of Velum; 

 B, Embryonic Muco-Cartilage op Tentacu- 

 lar Bar. 



