14 ILXPLAKATION OF 1HE FK.UUUS. 



Fro. 



77. A polyp, {Tiibularia incUvisa ;) m, nioutli ; o, ovarits ; p. ^^'. vcle% 



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, {Pohjcera ilhmiinata) 



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 cut'i 



h. blood-layer ; c,epidermis ; g, gland imbedded in the fat-layer,(y. 

 9-5. Egg of a skate-fish, {Myliobatis.) 

 98. Egg of hydra, 



97. Egg 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; 5, sero is or nervoa 



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, {rodurclla.) 



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

 embryo of the white-fish, magnified. 



123. Ycung 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. 

 120. The same more fully developed. The allantois (a) '^ further de- 



