102 DESCRIPTIONS OF PREPARATIONS. 



placed, bend outwards into this same septum. It is possible, therefore, that 

 the ribs of Elasmobranchii have undergone a change of position. 



Schmid-Monnard has recently investigated the origin of bony tissue in Teleostei. 

 He finds as follows, (i) The first bone is always formed outside the cartilage. 

 (2) All skeletogenous tissues may take part subsequently in the formation of 

 bone, and whether cartilage or connective tissue they ossify either directly or 

 indirectly, i.e. by means of osteoblasts. In most parts of the skeleton bone is 

 derived exclusively from perichondriifm. (3) The first-formed bone is invariably 

 homogeneous, containing neither bone-cells nor connective fibres. Bone-corpuscles 

 are to be found here and there in those fish in which, according to Kolliker, 

 they are absent, and the bone resembles dentine, e. g. Pike, Perch, Lota vulgaris, 

 Gadus aeglefinus, &c. But true dentine, i. e. tubular dentine, appears never to 

 be formed. The great development and regular arrangement of Sharpey's fibres 

 probably led to the supposition, coupled with the then imperfect methods of 

 research at command. 



Formation of vertebrae, arches and ribs in Teleostei, Gotte, A. M. A. xvii. 

 1879 ; Grassi, M. J. viii. 1882 ; in Ganoidei and Elasmobranchii, Gotte, A. M. A. 

 xv. 1878; in Elasmobranchii, Balfour, Elasmobranch Fishes, London, 1878; in 

 Lepidosteus, Id., and W. N. Parker, Ph. Tr. 173, 1882. 



For discussion as to homologies of ribs and formation of caudal canal, see work 

 last quoted. 



Azygosfins. Mayer, Mitth. Zool. Stat. Naples, vi. 1885. 



Formation of bone. Schmid-Monnard, Z. W. Z. xxxix., 1883, and Kolliker, 

 P. R. S. ix. 1859. Cf. Kostler, Z. W. Z. xxxvii. 1882. 



19. ASCIDIAN (Ascidia affinis), 



Dissected to show the chief features in the anatomy of Urochorda. 



THE animal is mounted with the base of attachment or posterior end 

 downwards, and the oral or inhalent aperture or anterior end upwards. The 

 ventral edge is on the left hand, the dorsal on the right, and the animal's 

 left side is therefore turned to the observer. On the dorsal edge, and some- 

 what anteriorly, is situated the atrial or exhalent aperture, through which a 

 white bristle has been passed. 



There is an external transparent test. This test is secreted by the 

 ectoderm cells, and is remarkable among animal tissues for containing 

 cellulose or a substance very closely akin to it. It is prolonged inwards for 

 a short distance at both oral and atrial apertures. The body walls, often 

 termed mantle, have shrunk away from the test under the action of the 

 alcohol in which the animal is preserved. This contraction does not take 

 place in all species of the genus, and in this instance the body walls remain 

 firmly united to the test in the region of the two apertures. They may be 

 seen to be fibrillated, and consist of connective tissue with bundles of 

 muscular fibres. In the genus Ascidia these bundles are more or less 

 irregularly arranged, and interlace more or less with one another, a fact to 

 be made out easily in the part of the body walls left near the atrial aperture. 



