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CLEAVAGE AND SEGMENTATION 67<br />
PLATE XLI<br />
Plate xli. illustrates cleavage or segmentation in animals ; also, in a minor degree, radiating and concentric<br />
arrangements.<br />
Fig. 1.—Dissection of earthworm, by Masterman—dorsal view. Shows transverse divisions and septa.<br />
A. Superficial dissection, a, Pharyngeal muscles ; b, pharynx ; t, lateral hearts ; d, cesophageal glands ; e, dorsal blood-vessel<br />
/, crop ; g, gi/.zard ; h, intestine.<br />
B. Deep dissection with alimentary canal, &c., removed, i, Brain ; j, septa ; k, nephridia ; I, nerve cord ; m, spermatheca ;<br />
n, seminal vesicle ; o, ovary ; p, vas deferens ; q, oviduct.<br />
C. Transverse section of earthworm in intestinal region. Shows radiating and concentric arrangements, a; Cuticle ; b, ectoderm ;<br />
c, circular muscles ; d, longitudinal muscles ; /, yellow cells ; g, dorsal pore ; h, dorsal blood-vessel ; i, typhlosole ; j, intestine ;<br />
k, peri-visceral cavity ; I, nerve cord ; m, ventral blood-vessel.<br />
Fig. 2.—Views of adult and young tapeworm {Teenia solium), (after Leuckart). Show segmentation, radiating, branched, and<br />
concentric arrangements.<br />
A. Body of adult Tsenia segmented transversely {Txnia saginata).<br />
B. Head of Tsenia with circlet of hooks (a), four suckers (h), and primary segments (c).<br />
0. Semi-diagrammatic view of a single proglottis of Tsenia. a, Ovary ; b, testis ; c, receptacle ; d, seminal vesicle ; e, common<br />
sexual aperture ; /, uterus ; g, yolk-gland ; h, shell-gland ; i, excretory canal ; j, nerve cord.<br />
D. E, P, G, H. Development of Txnia solium.<br />
Pig. 3.—Diagrammatic, median, longitudinal section of an ascidian by Masterman. Shows longitudinal and transverse division.<br />
a, Mouth ; b, buccal cavity ; c, peri-pharyngeal groove ; d, pharyngeal clefts ; e, endostyle ; /, pharyngeal part of ventral vessel<br />
g, heart ; h, gonad ; i, nerve ganglion ; j, subneural gland ; k, atriopore ; I, intestine ; m, genital duct ; n, atrium ; o, dorsal bloodvessel<br />
; ^, test (cuticle) ; q, mantle (ectoderm) ; r, stomach ; s, intestinal part of ventral vessel.<br />
Pig. 4.—A. The common crayfish (Astacus fluviatilis, male), (after Huxley). Natural size. Shows radiation, branching, and<br />
segmentation in the limbs, claws, &c., and transverse division and segmentation in the body.<br />
B. Dissection of the same from above, to show the central nervous system, &c. a, Supra-cesophageal ganglion ; b, infra-cesophageal<br />
ganglion ; e, fifth thoracic ganglion ; d, last thoracic ganglion ; c, last abdominal ganglion ; /, optic nerve ; g, antennulary nerve ;<br />
/;., antennary nerve ; i, stomato-gastric nerve ; j, circum-oesophageal commissures ; k, cesophagus in cross section ; I, vent.<br />
thereof.<br />
PLATE XLII<br />
Plate xlii. illustrates spiral, longitudinal, radiating, and transverse division in plants and animals, and parts<br />
Pig. 1.—Photograph of plane tree (Acer 2Keudoplaiarms) growing at Deans Court, St. Andrews, N.B. Sho\\s spiral, longitudinal<br />
division of stem or bole. This is a rather common arrangement in the trunks of trees generally. The spirals may be right or lefthanded.<br />
Similar spiral arrangements are seen in water-spouts (Plate vi., p. 13) ; the stems of many plants (Plate x.. Figs. 1, 2, and<br />
3) ; various shells (Plate xv., Fig. 1) ; horns (Plate xv.. Fig. 2 ; Plate xviii., Fig. 2) ; tusks (Plate xvi., Pig. 2) ; the bones of animals,<br />
especially the bones of the extremities (Plate xx., Fig. 3 ; Plate xxi., Fig. 1) ; the umbilical cord, and certain intestines (Plate xii.,<br />
Figs. 3 and ; 4) the muscles of the left ventricle of the heart (Plate xvii.. Fig. 4), &c. The spiral, it will be seen, is no chance product<br />
in nature (the Author).<br />
Pig. 2.—General views of the lajxrs of the retina of the human eye (after Max Schultze). Show longitudinal and transverse<br />
division.<br />
A. The layers magnified 400 diameters, a, Membrana limitans interna ; 6, optic fibre layer ; c, ganglion-cell layer ; d, internal<br />
granulated (molecular) layer ; e, internal granule layer ; /, external granulated (intergranule) layer ; g, external granule layer, including<br />
the external fibre layer ; h, membrana limitans externa ; i, layer of rods and cones ; j, pigment layer.<br />
B. The posterior part of the fundus of the human retina magnified 800 diameters. /, External granulated layer ; g, external<br />
granule layer ; /(, limitans externa ; i, rods and cones ; the supporting connective tissue is omitted in this figure.<br />
C. Cone and cone fibre, the latter presenting varicosities : from<br />
Magnified 500 diameters.<br />
the vicinity of the yellow spot of the retina of the human eye.<br />
Fig. 3.—Cuttlefish (Sepia officinalis). Shows longitudinal, slightly radiating division and transverse markings.<br />
A. Dorsal view of cuttlefish. B. Ventral view of cuttlefish.<br />
Fm, 4.—Vertical section of sea anemone, copied from a water-colour drawing by C, Berjeau in the Zoological Department<br />
of the British Museum. Shows radiating, branched arrangement, a, Non-pinnate tentacles, the cavities of which communicate with<br />
the general cavity (6), which is divided into compartments by septa (c), on the walls of which the gonads (d) are developed. The axis<br />
is occupied by the stomach (c), which communicates below with the general cavity (1j) and opens above by a mouth marked by a special<br />
slit (/). g, Marks the point at which a chamber is in communication with its neighbour, and h is the lower surface of the disc.<br />
FtG. 5.—Fibrils of voluntary striped muscle in the uncontracted and contracted states, as figured by Professor Rutherford. Show<br />
longitudinal and transverse cleavage.<br />
A. Fibrils of extensor muscle of crab's leg, uncontracted and fully extended.<br />
B. Fibrils of muscle of crab's leg, uncontracted and unextended.<br />
C.<br />
D.<br />
Bundle of semi-contracted fibrils from muscle of crab's leg.<br />
A further stage of contraction of muscle of crab's leg.<br />
E. Fully contracted muscle of crab's leg, showing complete reversal of stripes.<br />
F. Contracted muscle from same preparation as that figured at E.<br />
G. A bundle of contracted fibrils.<br />
, ^ , , , •., . ^ i i t ,<br />
The sarcous elements of the fibrils, as the author explained in 1872-3 (Edm. Med. Jour.), are endowed with a centripetal and centrifugal<br />
power, whereby they can elongate in the direction of the length or breadth of the fibril, according as the fibril is to be lengthened or<br />
shortened in the so-called relaxation and contraction of muscle. The original diagrams illustrating this point are given at Plate<br />
Ixxxiii., p. 320.