. Cave vertebrates of America; a study in degenerative evolution. Cave animals; Evolution. THE MIGRATION OF THE ANUS. 95 There is a regular change in the position of the embryo with development. The blastoderm is formed at the side of the yolk. When the gastrula covers half the yolk, the egg has rotated so that the gastrula covers more of the lower than of the upper surface of the yolk. Still later, some hours before the closing of the blastopore the latter structure lies to one side of the yolk-sphere, which always occupies the upper pole of the egg; the embryo extends from this region obliqu
. Cave vertebrates of America; a study in degenerative evolution. Cave animals; Evolution. THE MIGRATION OF THE ANUS. 95 There is a regular change in the position of the embryo with development. The blastoderm is formed at the side of the yolk. When the gastrula covers half the yolk, the egg has rotated so that the gastrula covers more of the lower than of the upper surface of the yolk. Still later, some hours before the closing of the blastopore the latter structure lies to one side of the yolk-sphere, which always occupies the upper pole of the egg; the embryo extends from this region obliquely over the yolk. After the formation of the tail the embryo is always found coiled about the upper half of the yolk. The period spent in the egg lasts about a month. In the laboratory some embryos hatched in about 28 days, but in the cold cave streams this period would probably be several days longer. The yolk has been but little affected at the time of hatching, measuring mm., the oil-sphere about I mm.; and since the yolk is all absorbed before the young are freed from the gill membrane, probably another month is spent under the gill Fig. 30. (a) Internal Anatomy oi Amblyopsisspeltrus. i, anus; 2, opening of oviduct; 3, oviduct; 4, ovary, which is single; 5, liver; 6, duodenum; 7, gali sac; 8, pectoral tin; p. one of pyloric ca^ca; ro, caecum; 11, stomach; 12. spleen; 13, air bladder; 14 and 16, intestine; i^, pancreas; /., liver. Alimentary Canal of Clwlogastcr conmtus. pc, pyloric cseca; s., stomach; 1 Alimentary Canal of Chclogaster papilli/crus. Alimentary Canal of Clwlogasler agassizii. (e) Alimentary Canal of Typhluhthys sub/crraneus. The young, on hatching, are about 5 mm. long and lie on their sides. The motion of the tail produces no effect other than to cause them to spin around with the yolk for a pivot. The metamorphosis of the larva into the definitive fish is completed before it leaves the gill cavity of the mother. The longest individ- uals I h
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