47 



PLATE XXIII. 



THE FROG" continued. 



DEVELOPMENT 



FIG. 1. EGGS, natural size (see PI. XXII. fig. 12, Egg magnified) 



a. Ovarian ova. 



b. Laid eggs, surrounded by successive layers of albumen or white of egg secreted by the oviduct 



and swelling up in water. 

 FIG. 2. IMPREGNATED EGG showing successive stages in the process of yelk-division 



The immediate result of this process is the formation of a morula (/), which by the soaking in 



of fluid into its interior becomes a hollow sphere. 

 DIAG. 1. DEVELOPMENT OF TADPOLE 



f Outer layer or epiblast. 

 a. Blastoderm or germinal membrane -j Middle layer or mesoblast. 



(. Inner layer or hypoblast (blue). 

 Medullary groove, a median longitudinal depression. 

 Laminae dorsales (L. dorsal plates) or medullary folds, the epiblust raised up into folds on 



each side of medullary groove. 



Notochord or chorda dorsalis, the primitive axial column. 



d. Neural canal formed by the union of the dorsal lamina), with neural arches in its walls. 

 Centra of spinal column invest notochord, which persists both in the centres of the bodies 



of the vertebras and in the urostyle. 

 /. Spirally-coiled intestine. 

 Large muscular tail. 



Chief phases in life of Tadpole shown in PI. XXII. 

 FIG. 3. Perfect Frog, tailless, in natural position. 

 CLASSIFICATION 



Sub-Kingdom Vertebrata, because it possesses a vertebral column ; limbs with an internal 



skeleton ; and a portal vein with a capillary network at both its ends, receiving the 



capillaries of the alimentary canal, and distributing to the capillaries of the liver. 



Class Amphibia (Gr. amphi, both ; bios, life), because as a Tadpole it has gills, afterwards 



lungs ; two condyles on the skull for articulation with the vertebral column ; and a cloaca. 



Natural Order Anou'ra (Gr. a, without ; oura, a tail), because devoid of tail and gills in 



adult life. 

 Genus Rana. 

 Common Name Frog. 



Comparative Histology of Frog and Man. 



FIGS. 4, 4a. BLOOD-COEPUSCLES 



f Frog large, oval, and nucleated. 

 \ Human smaller, round, and non-nucleated. 

 Colourless, like amoebae (see PL IX. fig. 4). 

 FIGS. 5, 5a. EPITHELIUM (Gr. epi, upon ; thallo, I grow) in its principal varieties 



a. Squamous (L. squama, a scale) or scaly. 



b. Columnar or cylindrical. 



c. Ciliated. 



Spheroidal or glandular (see PI. XXII. fig. 11). 

 FIG. G. a, b, c. CONNECTIVE TISSUE in its two principal varieties 



TIT m f Connective-tissue corpuscles, nucleated cells. 



WHITE FIBROUS TISSUE .' , 



( Matrix, fibrous. 



YELLO\V ELASTIC TISSUE, resisting acetic acid. 



FIG. 7 and DIAG. 2. VERTICAL SECTION OF EPIDERMIS OB EPITHELIUM. 

 FIG. 8. a, b. CARTILAGE or GRISTLE 



p .. ( Matrix, granular. 



\ Cartilage- cells, nucleated. 



