68 



OVUM. 



gradually acquires the peculiar dry hardness 

 which characterises it after the egg is laid.* 

 The view of H. Meckel that the animal basis 

 of the shell is formed by the separation of 

 a layer of the mucous membrane of the ute- 

 rine part of the oviduct does not appear to 

 be established. 



During the time that the shell is forming, 

 the distinction between the softer and thinner 

 external albumen, and the more dense and 

 deeper part, becomes more obvious, and, at 

 the same time, according to M. Coste, a cer- 

 tain degree of liquefaction occurs in a layer 

 of albumen immediately surrounding the yolk, 

 which allows the latter body to float more 

 freely within the superincumbent albumen. 



The egg remains in the uterine dilatation 

 till it is about to be laid. The expulsion of 

 it from this cavity through .the narrow part 

 of the tube, leading into the cloaca, requires 

 very strong muscular contraction for its ac- 

 complishment ; and, although the egg always 

 descends in the oviduct, and usually lies in 

 the uterus, with its narrow end downwards, 

 both Purkinje and Von Baer state that they 



have sometimes seen its position inverted 

 towards the end of the time of its residence 

 there in consequence of the force of the mus- 

 cular contractions of the wall of the oviduct. 



Ovarian ovum of birds ; ovulum ; yolk and its 

 contents. The yolk, yelk, or vitellus (Jaime, 

 Fr. Dotter, Germ.) consists in the newly 

 laid egg of the external enclosing vitelline 

 membrane, of the yolk substance, a mass of 

 vesicular, cellular, and granular matter of va- 

 rious structure, to which as a whole the 

 membrane gives a subglobular form, and on 

 the surface of this mass, below or within the 

 vitelline membrane, and on that side of the 

 yolk which naturally turns uppermost in the 

 complete egg, the cicatricula, or embryo spot, 

 a thin disc of organised cellular structure, in 

 which, under the influence of heat and air, 

 as during ordinary incubation, the embryo, 

 and its accompanying foetal membranes, &c., 

 are first formed. 



The cicatricula of the laid egg, as has al- 

 ready been remarked, however, has, during its 

 descent through the oviduct, undergone some 

 part of those changes which belong to the 



Fig. 49. 



Form of the Fowl's egg and structure of the yolk as exhibited by a section. 



A. Sectional view of the fowl's egg ; a, yolk enclosed by its vitelline membrane ; b, //, inner and 

 outer parts of the albumen : c, c, chalazae ; d, two principal layers of tbe lining membrane of the 

 shell ; e, calcareous shell ; /, air-space between the two layers of the shell membrane. 



B. Outline of the yolk ; , cicatricula ; b, nucleus of the cicatricula ; c, yolk cavity or latebra, and 

 canal ; d, concentric deposits of yolk substance or halones ; m, vitelline membrane. 



fecundated condition, and by which the found- 

 ation is laid of that structure in which the 

 future embryo is more immediately developed ; 

 for it has now lost its germinal vesicle, and 

 from being formed, as at first, of mere granules 

 or simple spherules, it has acquired a true 

 organised cellular structure. It now consists, 

 in fact, of the delicate discoid collection of 

 cells, which has been called blastoderma. It 

 may be proper, therefore, to consider the mass 

 of the yolk and the germ, in their unfecundated 

 state, while still within the ovarian capsule, 



* It is to be remarked that the animal basis of 

 the calcareous shell is of quite a different structure 

 from the fibrous lining membrane of the shell ; and 

 the calcareous deposit is not to be regarded as 

 taking place in that fibrous membrane. The outer- 

 most layer of the lining membrane adheres very 

 firmly to the shell, which may have misled some 

 on this point, who describe the animal basis of the 

 calcareous shell as of the same structure with the 

 fibrous lining membrane. 



next, after the ovulum has entered the ovi- 

 duct, and, subsequently, when it is laid ; 

 reserving, however, for a latter part of the 

 article the account of the process by which 

 the change in the cicatricula referred to takes 

 place. 



In the newly laid egg the yolk forms an 

 ellipsoidal mass, somewhat flattened on the 

 upper or cicatricular surface, and with its 

 long axis corresponding to that of the egg. 

 Its largest diameter is about one inch and a 

 quarter, its shortest about an inch : it floats 

 within the white, capable of a certain degree 

 of motion, which is controlled, as before ex- 

 plained, by its own specific gravity, and by 

 the attachment of the chalazae. 



The yolk substance is not of the same 

 nature throughout, there being a part of a 

 lighter colour in the centre, about one fourth 

 of the diameter of the whole; from this, a 

 narrower prolongation extends upwards 



