. The chordates. Chordata. 236 Basic Structure of Vertebrates. MjP Fig. 199. Amphibian eggs. (A) Of frog, soon after laying. (B) Early larva of frog, just before hatching. (C) Of the salamander, Cryptobranchus allegheniensis. (G) Gelatinous layer; (L) larva; (OV) ovum. (A and C, approximately actual size; B, enlarged.) (A and B, after Marshall; C, after Beese. Courtesy, Neal and Band: "Chordate Anatomy," Philadelphia, The Blakiston Company.) Eggs of mammals, with two exceptions, are minute, containing a minimum of yolk. The exceptions are the duckbill (Ornifhorhynchus) and the spiny
. The chordates. Chordata. 236 Basic Structure of Vertebrates. MjP Fig. 199. Amphibian eggs. (A) Of frog, soon after laying. (B) Early larva of frog, just before hatching. (C) Of the salamander, Cryptobranchus allegheniensis. (G) Gelatinous layer; (L) larva; (OV) ovum. (A and C, approximately actual size; B, enlarged.) (A and B, after Marshall; C, after Beese. Courtesy, Neal and Band: "Chordate Anatomy," Philadelphia, The Blakiston Company.) Eggs of mammals, with two exceptions, are minute, containing a minimum of yolk. The exceptions are the duckbill (Ornifhorhynchus) and the spiny anteater (Echidna) of the Australian region. These two mammals, presumably of primitive type, lay large eggs encased in tough shells. In general, these mammals are reptilian in their methods of reproduction. Otherwise, mammalian eggs are of microscopic dimen- sions ( to mm. in diameter). The egg (Fig. 201) is covered by a delicate membrane (zona pellucida), external to which may be a cellular membrane (corona radiata), both contributed by the ovary. Fertilization. Development is initiated by the "fertilization" of the egg. A spermatozoon penetrates the egg (impregnation) and the neck of Utebra. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Rand, Herbert W. (Herbert Wilbur), 1872-1960. Philadelphia : Blakiston
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