File:The biology of the frog (1907) (14760788271).jpg

Identifier: biologyoffrog1907holm (find matches)
Title: The biology of the frog
Year: 1907 (1900s)
Authors: Holmes, Samuel J. (Samuel Jackson), 1868-
Subjects: Frogs
Publisher: New York : Macmillan
Contributing Library: Harvard University, Museum of Comparative Zoology, Ernst Mayr Library
Digitizing Sponsor: Harvard University, Museum of Comparative Zoology, Ernst Mayr Library

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wever, a single organ, and their ducts later becomeconnected with the bile duct. Only the secreting portionof the pancreas and the lining of its ducts are of entodermicorigin, the connective tissue, blood vessels, etc., arising, asin the liver, from the middle germ layer. The bladder arises as an outgrowth of the ventral sideof the alimentary canal, near the posterior end; its lining,therefore, is of entodermic origin. The lungs appear as a pair of pouches from the sides ofthe esophagus. They make little growth until quite late inthe life of the tadpole. The region of the esophagus fromwhich the lungs arise becomes depressed and partly sepa-rated off from the part above to form the larynx, the mouthof the depressed portion going to form the glottis of theadult. The gill slits in the frog appear in the form of five solidoutgrowths on each side of the anterior portion of thearchenteron. In section they are shown to be in the formof a double fold such as would be produced if the walls of
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chap, v THE DEVELOPMENT OF THE FROG 109 a pouch-like diverticulum were brought into contact. Attheir outer ends the slits come into contact with the ecto-derm, with which they fuse. The first two slits are the firstto form; the others appear in order from before backward.When the tadpole escapes from the jelly into the water, thewalls of the solid gill slits separate, the ectoderm breaksthrough at the outer end, and a free communication is estab-lished between the throat and the outside. The first slit,the hyomandibular, does not break through to the outside ;its entodermic lamellae separate and form a pouch whichcommunicates with the pharynx. In most forms the hyo-mandibular cleft forms the Eustachian tube and its coveringover its outer end, the tympanic membrane ; but in the frog,according to Marshall, the Eustachian tube has a differentmethod of origin. (See Vertebrate Embryology, p. 143.)The four following slits are known as the branchial clefts;of these the second and third open

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