14 EXPLANATION OF THE FIGURES. 



FlG. 



77. A polyp, (Tiibularia indivisa ) m, mouth ; o, ovaries ; p, tentacles. 



78. Blood disks in man, magnified. 



79. " " in birds, " 



80. " " in reptiles, " 



81. " " in fishes, " 



82. Portion of a vein opened, to show the valves. 



83. Network of capillary vessels. 



84. Dorsal vessel of an insect, with its valves. 



85. Cavities of the heart of mammals and birds. 



86. " " " of a reptile. 



87. " " " of a fish. 



88. Heart and bloodvessels of a gasteropod mollusk, (Natica.) 



89. Tracheae, or air tubes of an insect ; s, stigmata ; t, trachea. 



90. Relative position of the heart and lungs in man. 



91. Respiratory organs of a naked mollusk, (Polycera illuminata.) 



92. Respiratory organs (gills) of a fish. 



93. Vesicles and canals of the salivary glands. 



94. Section of the skin, magnified, to show the sweat glands ; a, the cutis ; 



b, blood-layer ; c, epidermis ; g, gland imbedded in the fat-layer^/.) 



95. Egg of a skate-fish, (Myliobatis.) 



96. Egg of hydra. 



97. Ege of snow-flea, (Podurella.) 



98. Section of an ovarian egg ; d, germinative dot ; g, germinative vesi- 



cle ; s, shell membrane ; v, vitelline membrane. 



99. Egg cases of Pyrula. 



100. Monoculus bearing its eggs, a a. 



101. Section of a bird's egg ; a, albumen ; c, chalaza ; e, embryo ; s, shell ; 



y, yolk. 



102. Cell-layer of the germ. 



103. Separation of the cell-layer into three layers ; s, serous or nervous 



layer; m, mucous or vegetative layer; v, vascular or blood layer. 



104. Embryo of a crab, showing its incipient rings. 



105. Embryo of a vertebrate, showing the dorsal furrow. 



106-8. Sections of the embryo, showing the formation of the dorsal canal. 



109. Section, showing the position of the embryo of a vertebrate, in re- 



lation to the yolk. 



110. Section, showing the same in an articulate, (Podurella.) 

 111-22. Sections, showing the successive stages of development of the 



embryo of the white-fish, magnified. 

 123. Young white-fish just escaped from the egg, with the yolk not yet 



fully taken in. 

 124, 125. Sections of the embryo of a bird, showing the formation of the 



allantois ; e, embryo ; x x, membrane rising to form the amnios ; 



a, allantois ; y, yolk. 

 126. The same more fully developed. The allantois (a) is further de- 



