CLEAVAGE AND SEGMENTATION 67 



PLATE XLI 



Plate xli. illustrates cleavage or segmentation in animals ; also, in a minor degree, radiating and concentric 

 arrangements. 



Fig. 1.— Dissection of earthworm, by Masterman— dorsal view. Shows transverse divisions and septa. 



A. Superficial dissection, a, Pharyngeal muscles ; b, pharynx ; t, lateral hearts ; d, cesophageal glands ; e, dorsal blood-vessel ; 

 /, crop ; g, gi/.zard ; h, intestine. 



B. Deep dissection with alimentary canal, &c., removed, i, Brain ; j, septa ; k, nephridia ; I, nerve cord ; m, spermatheca ; 

 n, seminal vesicle ; o, ovary ; p, vas deferens ; q, oviduct. 



C. Transverse section of earthworm in intestinal region. Shows radiating and concentric arrangements, a; Cuticle ; b, ectoderm ; 

 c, circular muscles ; d, longitudinal muscles ; /, yellow cells ; g, dorsal pore ; h, dorsal blood-vessel ; i, typhlosole ; j, intestine ; 

 k, peri-visceral cavity ; I, nerve cord ; m, ventral blood-vessel. 



Fig. 2.— Views of adult and young tapeworm {Teenia solium), (after Leuckart). Show segmentation, radiating, branched, and 

 concentric arrangements. 



A. Body of adult Tsenia segmented transversely {Txnia saginata). 



B. Head of Tsenia with circlet of hooks (a), four suckers (h), and primary segments (c). 



0. Semi-diagrammatic view of a single proglottis of Tsenia. a, Ovary ; b, testis ; c, receptacle ; d, seminal vesicle ; e, common 

 sexual aperture ; /, uterus ; g, yolk-gland ; h, shell-gland ; i, excretory canal ; j, nerve cord. 



D. E, P, G, H. Development of Txnia solium. 



Pig. 3. — Diagrammatic, median, longitudinal section of an ascidian by Masterman. Shows longitudinal and transverse division. 

 a, Mouth ; b, buccal cavity ; c, peri-pharyngeal groove ; d, pharyngeal clefts ; e, endostyle ; /, pharyngeal part of ventral vessel ; 

 g, heart ; h, gonad ; i, nerve ganglion ; j, subneural gland ; k, atriopore ; I, intestine ; m, genital duct ; n, atrium ; o, dorsal blood- 

 vessel ; ^, test (cuticle) ; q, mantle (ectoderm) ; r, stomach ; s, intestinal part of ventral vessel. 



Pig. 4. — A. The common crayfish (Astacus fluviatilis, male), (after Huxley). Natural size. Shows radiation, branching, and 

 segmentation in the limbs, claws, &c., and transverse division and segmentation in the body. 



B. Dissection of the same from above, to show the central nervous system, &c. a, Supra-cesophageal ganglion ; b, infra-cesophageal 

 ganglion ; e, fifth thoracic ganglion ; d, last thoracic ganglion ; c, last abdominal ganglion ; /, optic nerve ; g, antennulary nerve ; 

 /;., antennary nerve ; i, stomato-gastric nerve ; j, circum-oesophageal commissures ; k, cesophagus in cross section ; I, vent. 



PLATE XLII 



Plate xlii. illustrates spiral, longitudinal, radiating, and transverse division in plants and animals, and parts 

 thereof. 



Pig. 1. — Photograph of plane tree (Acer 2Keudoplaiarms) growing at Deans Court, St. Andrews, N.B. Sho\\s spiral, longitudinal 

 division of stem or bole. This is a rather common arrangement in the trunks of trees generally. The spirals may be right or left- 

 handed. Similar spiral arrangements are seen in water-spouts (Plate vi., p. 13) ; the stems of many plants (Plate x.. Figs. 1, 2, and 

 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, 

 especially the bones of the extremities (Plate xx., Fig. 3 ; Plate xxi., Fig. 1) ; the umbilical cord, and certain intestines (Plate xii., 

 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 

 in nature (the Author). 



Pig. 2. — General views of the lajxrs of the retina of the human eye (after Max Schultze). Show longitudinal and transverse 

 division. 



A. The layers magnified 400 diameters, a, Membrana limitans interna ; 6, optic fibre layer ; c, ganglion-cell layer ; d, internal 

 granulated (molecular) layer ; e, internal granule layer ; /, external granulated (intergranule) layer ; g, external granule layer, including 

 the external fibre layer ; h, membrana limitans externa ; i, layer of rods and cones ; j, pigment layer. 



B. The posterior part of the fundus of the human retina magnified 800 diameters. /, External granulated layer ; g, external 

 granule layer ; /(, limitans externa ; i, rods and cones ; the supporting connective tissue is omitted in this figure. 



C. Cone and cone fibre, the latter presenting varicosities : from the vicinity of the yellow spot of the retina of the human eye. 

 Magnified 500 diameters. 



Fig. 3.— Cuttlefish (Sepia officinalis). Shows longitudinal, slightly radiating division and transverse markings. 

 A. Dorsal view of cuttlefish. B. Ventral view of cuttlefish. 



Fm, 4. — Vertical section of sea anemone, copied from a water-colour drawing by C, Berjeau in the Zoological Department 

 of the British Museum. Shows radiating, branched arrangement, a, Non-pinnate tentacles, the cavities of which communicate with 

 the general cavity (6), which is divided into compartments by septa (c), on the walls of which the gonads (d) are developed. The axis 

 is occupied by the stomach (c), which communicates below with the general cavity (1j) and opens above by a mouth marked by a special 

 slit (/). g, Marks the point at which a chamber is in communication with its neighbour, and h is the lower surface of the disc. 



FtG. 5.— Fibrils of voluntary striped muscle in the uncontracted and contracted states, as figured by Professor Rutherford. Show 

 longitudinal and transverse cleavage. 



A. Fibrils of extensor muscle of crab's leg, uncontracted and fully extended. 



B. Fibrils of muscle of crab's leg, uncontracted and unextended. 



C. Bundle of semi-contracted fibrils from muscle of crab's leg. 



D. A further stage of contraction of muscle of crab's leg. 



E. Fully contracted muscle of crab's leg, showing complete reversal of stripes. 



F. Contracted muscle from same preparation as that figured at E. 



G. A bundle of contracted fibrils. , ^ , , , •., . ■ ^ i i t , 

 The sarcous elements of the fibrils, as the author explained in 1872-3 (Edm. Med. Jour.), are endowed with a centripetal and centrifugal 



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 

 shortened in the so-called relaxation and contraction of muscle. The original diagrams illustrating this point are given at Plate 

 Ixxxiii., p. 320. 



