. Elementary text-book of zoology. Zoology. 25-2 AYES. DM -t/-:H the male, there are also clitoris-like structures at the same place in the female. Development.—Birds are, without exception, oviparous (relation to power of flight). The egg is remarkable for the large amount of yolk (distinguishable into white and yellow yolk), and its porous calcareous shell (fig. 659). The development requires a high tem- perature, at least equal to that of the blood. The necessary heat is usually supplied by the bird during incubation. Fertilization takes place in the upper region of the oviduct before the s
. Elementary text-book of zoology. Zoology. 25-2 AYES. DM -t/-:H the male, there are also clitoris-like structures at the same place in the female. Development.—Birds are, without exception, oviparous (relation to power of flight). The egg is remarkable for the large amount of yolk (distinguishable into white and yellow yolk), and its porous calcareous shell (fig. 659). The development requires a high tem- perature, at least equal to that of the blood. The necessary heat is usually supplied by the bird during incubation. Fertilization takes place in the upper region of the oviduct before the secretion of the albumen and of the shell membrane, and is at once followed by the partial (discoidal) segmentation which only implicates the clear part of the EW y°lk (formative yolk) • KS around the germinal vesicle—the so-called tread of the cock (cicatricula). When the egg is laid, the segmenta- tion is already com- p 1 e t e d, and the cicatricula has de- veloped into the germinal disc or FIG. 059.—Diagrammatic longitudinal section through an un- bl((/s(0(le)>nl The em- developed hen's e£roiii. £S, calcareous shell; LR, air-chamber. developes, as in Rep- tiles, the characteristic foetal membranes—the amnion and allantois (fig. 635). The duration of the embryonic development varies according to the size of the egg and the relative development of the young when hatched. The Bird, when ready to creep out, breaks the blunt end of the shell by means of a sharp tooth placed at the extremity of the upper beak. The young when hatched have essentially the organisation of the adult animal, although they may still be far inferior to it in the degree of their bodily development. While the Gall mace i and the Cursores, and most Grallatores and X<i,fnt/»-es have when hatched a complete covering of down, and are so far advanced in development, that they at once follow the mother on land or into water and there seek their own food (prsecoces) ; others like the. Please n
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
