CHAPTER VII. 

 FORMATION OF THE EMBETO IN THE FOWL'S EGG. 



THE process of embryonic development in the egg of the bird differs 

 from that of the frog in two important particulars. First, the 

 whole of the vitellus, or yolk, in the bird's egg, is not directly converted 

 into the body of the embryo, but a large part is transformed into a 

 nutritious fluid, and thus serves indirectly for its growth. Secondly, 

 certain accessory organs make their appearance, extending beyond the 

 limits of the body of the embryo, and surrounding it with membranous 

 envelopes. The development of the chick, during incubation, has been 

 found especially favorable for the study of many details as to the 

 formation and growth of the various organs ; and some of the most 

 valuable discoveries in embryology have been obtained in this way. 



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

 something more than the vitellus proper. Its principal mass COD 

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

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

 This substance contains an abundance of soft, spherical, finely granular 

 bodies, from 25 to 100 mmm. in diameter. 



The yellow yolk is everywhere surrounded by a thin nearly colorless 

 layer, the " white yolk,' 1 which contains, instead of the granular spheres 

 above described, 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 stratum of this substance remains 

 semifluid. There is also a spot at the centre of the yolk, which is 

 occupied by the same material, and which consequently remains soft 

 in the boiled egg ; the cavity thus left communicating with the surface 

 of the yolk by a narrow passage, like the neck of a flask. 



The yolk is thus formed of two substances, distinguished by their 

 microscopic characters and by their comparative coagulability at the 

 boiling temperature. Neither of these substances corresponds with the 

 granular vitellus of the mammalian egg ; they constitute a deposit of 

 nutritious material, destined for the support of the embryonic tissues. 



At one point on the surface of the yolk, in the unfecundated egg, is 

 a whitish circular spot, about three millimetres in diameter, immediately 

 beneath the vitelline membrane. This is the cicatricula. It is a thin 

 layer, of minutely granular structure ; its granules being imbedded in 

 a homogeneous substance by which they are agglutinated into a disk- 

 like mass. In its centre is the germinative vesicle, distinctly visible by 



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