. Chordate morphology. Morphology (Animals); Chordata. ^ oral sucker operculum external gills mouth. remnant of oral sucker Figure 7-24. Some stages in the development of the frog larva. A, at hotching; B, 8-mm larva; C, 1 1 '/2-mm larva; D, ventrol view of head end of C to show beginning of opercular fold; E, opercular fold covering right bronchial chamber with left chamber still opening broadly behind it. (After Huettner, 1949) The larval forms of some tetrapods may be modified for direct development. Fine examples are observed among the frogs. The tail undergoes reduction, the limbs appear
. Chordate morphology. Morphology (Animals); Chordata. ^ oral sucker operculum external gills mouth. remnant of oral sucker Figure 7-24. Some stages in the development of the frog larva. A, at hotching; B, 8-mm larva; C, 1 1 '/2-mm larva; D, ventrol view of head end of C to show beginning of opercular fold; E, opercular fold covering right bronchial chamber with left chamber still opening broadly behind it. (After Huettner, 1949) The larval forms of some tetrapods may be modified for direct development. Fine examples are observed among the frogs. The tail undergoes reduction, the limbs appear at a very early stage, an operculum covering the gills may not develop, and, in fact, the gills may not appear. The young of the frog is one of the most interesting verte- brate larvae since it shows a certain parallelism to an as- sumed primitive type. The agnathous mouth is armed with horny teeth used for eating algae. Bony jaws develop later, replacing the original mouthparts. There is an operculum- enclosed branchial chamber which could be considered as a kind of atrium. This atrium opens on the left side of the body by a single opening. The asymmetry of this larva is suggestive of that observed in the protochordates. The general opinion is that the frog larva is highly spe- cialized rather than primitive. The fact that the anuran as an adult is greatly modified does not necessarily imply that its larva has not retained basic agnathous or even prea- Protochordate 1. Sperm enters near vegetal pole. 2. Mesoderm mostly from enterocoelous pouches. 3. Notochord directly of archenteron wall origin. 4. Blastopore converted by ventral blastopore lip growth into anterior neuropere. Gut and neural canal connected. 5. Neural tube formed by rolling upward of margins from rear to front. 6. Atrium formed by invagi- nation of bilateral pouches or outgrowth of bilateral folds. 7. Gastrulation by simple invagination. 8. No neural crest. Vertebrate 1. Sperm enters near animal pole. 2. Only a
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Keywords: ., bookcentury1900, bookcollectionameri, bookcollectionbiodiversity
