CHAPTEE VIII. 



FORMATION OF THE EMBRYO IN THE FOWL'S EGG. 



IN the preceding chapter a condensed description has been given of 

 the general phenomena of embryonic development, as illustrated in the 

 egg of the frog This species is useful as an example, to exhibit the 

 progressive alterations of form which lead to the final production of 

 a vertebrate animal out of the fecundated vitellus, uncomplicated by the 

 presence of any accessory organs. But the development of the chick, in 

 the egg of the fowl during incubation, has been found more favorable 

 for the study of certain important details. The readiness with which the 

 fowl's egg may be obtained in all the successive stages of incubation, 

 and the convenient size of the embryo in the earlier periods of its forma- 

 tion, have made it a favorite subject of investigation for embryologists ; 

 and some of the most valuable discoveries in this department of physi- 

 ology have resulted from observations upon the young chick and the 

 mode of formation of its different organs. 



The Yolk and the Cicatricula. The yolk of the fowl's egg represents 

 something more than the vitellus proper. Its principal mass consists 

 of an opaque, yellow, semifluid substance, the "yellow yolk," which 

 solidifies on boiling, owing to its large proportion of albuminous matter. 

 This substance contains a great abundance of soft, spherical, finely 

 granular bodies, from 25 to 100 mmm. in diameter. 



The yellow yolk is surrounded everywhere by a thin layer of nearly 

 colorless appearance, the " white yolk," which contains, instead of the 

 granular spheres described above, smaller globular bodies with one or 

 more brightly refracting masses in their interior. The albuminous 

 matter of the white yolk, furthermore, does not solidify firmly on the 

 application of heat; so that in a boiled egg the thin outer stratum of this 

 substance remains semifluid. There is also a spot in the centre of the 

 yolk-sphere, which is occupied by the same material, and which conse- 

 quently remains soft in the boiled egg ; the central cavity thus left 

 communicating with the surface of the yolk by a narrow passage, like 

 the neck of a flask. 



The whole yolk is thus formed of two substances, which are distin- 

 guished from each other by their microscopic characters and by their 

 comparative coagulability at the temperature of boiling water. Neither 

 of the two corresponds with the granular vitellus of the mammalian egg ; 

 and the yolk, as a whole, constitutes a deposit of nutritious material, 

 superadded to the vitellus proper, and destined to be absorbed for the 

 support of the embryonic tissues. This yolk, however, is formed, in. 

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