This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
Fig. 369. Generative apparatus of the Hen.
* Vide Purkinje, Symbolic ad ovi Avium historiam ante incubationem. 4to. Lipsiae, 1830.
(2104). The phenomena attending conception are therefore simply these: - The membranes of the ovisac are gradually thinned by absorption; and being embraced and squeezed by the infundibular commence-ment of the oviduct, the transparent zone or stigma gives way, allowing the ovulum, covered only by its membrana vitelli, to escape into the ovi-ductus. The rent ovisac is soon removed by absorption; and the ovulum, with its cieatricula, is left to be clothed with other investments: but the germinal vesicle is now no longer to be seen; its delicate covering-having been, as Purkinje supposes, ruptured by the violence to which it has been subjected.
(2105). It is during the passage of the ovulum through the canal of the oviduct that it becomes enclosed in the other parts entering into the composition of the egg: these are, the albumen, the chalazce, the membrana putaminis, and the calcareous shell.
(2106). The albumen, or glairy fluid forming the white of the egg, is secreted by the mucous membrane that lines the commencement of the oviduct; and being laid on, layer upon layer, gradually coats the membrana vitelli. Some of the albumen meanwhile becomes inspissated, so as to form an almost invisible membrane, the chalazae, which, being-twisted by the revolutions of the yelk, as it is pushed forward in the oviduct, is gathered into two delicate and spiral cords (fig. 371, c c), whereby the yelk is retained in situ after the egg is completed.
(2107). The ovulum, now covered with a thick coating of albumen, and furnished with the chalazae, at length approaches the terminal extremity of the oviduct, where a more tenacious material is poured out: it is here that the whole becomes encased in a dense membrane resembling very thin parchment, called "membrana putaminis;" and ultimately, on arriving in the last, dilated portion of the canal (fig. 369, g), the lining membrane of which secretes cretaceous matter, the shell is formed by the gradual accumulation of extremely minute, polygonal calcareous particles, so disposed upon the surface of the egg that imperceptible interstices are left between them for the purpose of transpiration.
(2108). Thus, as the oviduct is traced from its infundibular commencement, the different portions of it are seen successively to discharge the following functions: - the orifice of the infundibulum receives the ovulum from the ovisae; the succeeding portion, extending nearly three-fourths of its entire length, secretes the albumen and the chalazae; in the next tract it furnishes the membrana putaminis; and in the last place, the shell; after which, the complete egg is expelled through the cloaca.
(2109). The anatomy of the egg prior to the commencement of incubation is therefore sufficiently simple. Immediately beneath the shell is the membrana putaminis; which, however, we must here remark, consists of two layers; and at the larger end of the egg these layers separate, leaving a space (fig. 370, a, b), called the vesicula aeris; we may further notice that the chamber so formed is filled with air containing an unusual proportion of oxygen, destined to serve for the respiration of the future embryo. Enclosed in the membrana putaminis the student next finds the albumen and chalazas (fig. 371, c); and lastly, the yelk, enclosed in its proper membrane (fig. 370, c), the membrana vitelli.
(2110). We must dwell a little more at length, however, upon the composition of the yelk. The cicatricula (fig. 370, g) is made up of a thin membrane, which originally enclosed the vesicle of Purkinje (f); but this latter, although introduced into the diagram for the purpose of illustration, is in reality, as we have already seen, no longer visible; and we must now change the word cicatricula for that of blastoderm, which may be presumed to consist of the original cicatricula and the ruptured vesicle of Purkinje: it is from this blastoderm, or germinal membrane, as it is sometimes called, that the future being is developed.
Fig. 370. Anatomy of the egg.
(2111). Immediately over the blastoderm the membrana vitelli is slightly thickened (fig. 370, h); and beneath it is a canal (e), which leads to a chamber (d) placed in the centre of the yelk; this cavity is filled with a whitish granular substance.
(2112). Such is the composition of the complete egg of a Fowl, and, with the exception of trifling circumstances hereafter to be noticed, of that of vertebrate animals in general. The development of the embryo is accomplished in the following manner.
(2113). No sooner has incubation* commenced, than the blastoderm becomes distinctly separate from the yelk and the membrana vitelli, and, as it begins to spread, assumes the form of a central pellucid spot, surrounded by a broad dark ring (fig. 371, g, h); it at the same time becomes thickened and prominent, and is soon separable into three layers: of these, the exterior (fig. 372, c) is a serous layer: the internal, or that next the yelk (a), a mucous layer; and between the two is situated a vascular layer (b), in which vessels soon become apparent. These three layers are of the utmost importance, as from the first-mentioned all the serous structures, from the second all the mucous structure, and from the third the entire vascular system of the embryo originate.
* Dr. Karl Ernst v. Baer, uber Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion. 4to. 1837.
Fig. 371. Egg at commencement of incubation.
Fig. 372. Earliest appearance of embryo.