File:The biology of the frog (1907) (14577497287).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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but the inner ones become elevated and archover the groove between them. Finally the two inner foldsmeet and fuse along the median line, converting the grooveinto a tube. The point where they first fuse correspondsto the neck region of the embryo ; and the closure of the tubeproceeds both forward and backward from this point. Thefusion extends backward so that folds on either side of theblastopore close in above that opening in such a way that itbecomes no longer visible from the outside. As the medul-lary tube is completed it is constricted off from the ecto-derm above, and the latter becomes continuous over themid-dorsal line. Subsequently it develops into the brainand spinal cord of the embryo. 1 King, Biol. Bull., Vol. 4, 1903. IOO THE BIOLOGY OF THE FROG CHAP. As the above changes are taking place the embryoelongates in the direction of the neural tube, which marksthe longitudinal axis of the future animal. On either sideof the anterior end of the neural tube there appears a pair
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FlG. 23.— Development of the embryo. A, yolk-plug stage; B, showingthe medullary folds, the blastopore nearly closed, and below the latterthe invagination which is to form the anus; C, D, later stages; E, themedullary folds have grown together and covered the blastopore. Abovethe anus is the rudiment of the tail. (From Morgan, after Ziegler.) of thickenings of the ectoderm. The anterior members ofeach pair, the sense plates, grow forward and meet in frontof the end of the neural tube; a depression appears in eachplate and marks the beginning of the ventral sucker of the THE DEVELOPMENT OF THE FROG IOI tadpole. Subsequently these depressions meet in front andbecome converted into a U-shaped groove. In the poste-rior pair of plates, the gill plates, there appear two verticalgrooves, which later become converted into the gill slits;later two additional slits appear, one before and one behindthe other two, but none of them break through until after

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