454 Mr. E. E. Prince on the 



mesoblastic perichordal investment is developed from the 

 innermost cells of the protovertebrje. This is tlie reduced 

 representative of the thick skeletogenous tube of Selachians 

 and Ganoids, and probably consists of a memhrcma elastica 

 interna only, though in favourable sections of young- Tele- 

 osteans two portions can be distinguished, a memhrana limitans 

 externa, which passes dorsally to enclose the neurochord 

 (constituting Eathke's memhrana reuniens superior) and 

 sends fibres down to meet below as the memhrana reuniens 

 inferior. It may be doubted whether the elastica externa^ 

 which separates the inner from the outer half of the skeleto- 

 genous layer in more primitive fishes, exists at all in Tele- 

 osteans, though in Mmtelus, the Rays, etc. this layer is much 

 nearer the chorda than in the HolocejoJiali, Notidanus, etc., in 

 which forms it is separated by a considerable interval from 

 the elastica interna. The outer layer, which we have distin- 

 guished as a limitans externa, may in reality be homologous 

 with the memhrana elastica externa, and certainly in the 

 species here considered cartilage develops, and McMurrich 

 observed * in Syngnathus a deposition of ossific matter 

 in this external layer. As the nucleated cartilage cells 

 arise they proceed outwards from the perichordal sheath as 

 two superior and two inferior rami (to each developing verte- 

 bral body), forming the neural and haimal arches respec- 

 tively. 



Cartilage appears to develop independently above the 

 medulla (in the memhrana reuniens superior), and the h^ma- 

 pophyses never really meet in the middle line, just as in the 

 cartilaginous sturgeons, but coalesce with the median dorsal 

 cartilage, which occupies the position of the longitudinal 

 elastic band between the distal ends of the upper rami, in 

 Acipenser, for example. 



Branchial Arches. 



The branchial arches are indicated as ridges passing dorso- 

 ventrally in the oesophageal region some hours before hatching. 

 They certainly remain closed for several days after their 

 appearance, and the clefts are not open to the exterior until 

 • the cartilaginous bars are developed. These bars can be 

 detected in course of development soon after the embryo has 

 emerged, the first aich developing so rapidly as to distort the 

 outline of the head. An anterior process (the maxillary) 

 passes immediately beneath the eyes ; but the hyoid arch, 

 being nearer the middle line, is less readily made out in the 



* Quart. Journ. Micr. Sci. 1883, p. 647. 



