EARLY EMBRYONIC DEVELOPMENTS. 1009 



serous ot germinal layer. The qualification of vascular laijer should not bo f^iven 

 to it, as it belongs to a special' formation which will be alluded to hereafter. 



3. Appearance and Formation of the Embryo.— The place in which the 

 embryo is developed is prepared by a diiferontiation in the germinal area. At 

 first opaque throughout its extent, it becomes clear in its central region, and is 

 thus divided into two concentric and circular zones — the travfiparent and the 

 opaqw zoii". The phenomena which follow are easily studied in the Chick, and 

 it is in it that we will observe them. But it is necessary at first to examine the 

 composition of a Bird's <i^g ; though its complexity is due to the circumstance 

 that the development of the embryo takes place externally, and that the germ 

 must therefore carry with it its nutriment and its protecti\e envelopes. 



Essential parts. — The Qgg of Birds — like that of Reptiles and all the Ovii)arous 

 Vertebrates — is, then, only a germ provided with an enormous quantity of aliment, 

 and it is this mass that constitutes the yolk. The germ itself — that which is the 

 physiological equivalent of the ovum of Mammals — is represented by a small, 

 circular, white spot — the cicatrinda — situated on the surface of the yolk, beneath 

 the vitelline membrane ; in this, exclusively, resides the evolutionary force, and 

 it is this spot which undergoes segmentation. It corresponds, then, to the whole 

 of the vitellus of the ovum in Mammalia, and merits the name of plastic vitellus, to 

 distinguish it from the nutritive vitellus represented by the yolk. Although there 

 is no marked line of demarcation between the two masses, and although from the 

 plastic vitellus there emanates a kind of protoplasmic atmosphere that penetrates 

 the nutritive vitellus, it might be maintained that the latter does not participate 

 to any extent in the segmentation, w^hich remains exclusively localized in the plastic 

 vitellus. In this sense it may be said that the segmentation is jmrtial in the ova 

 of the type named meroblastic. The ova of Mammalia and those of the same type 

 are, on the contrary, holohlastic — ova in which segmentation involves the whole 

 of the yolk. 



In the Fowl, whether or not fecundation has taken place, segmentation occurs 

 during the passage of the Qg^ in the tube, and is nearly completed at the moment 

 of laying. The cicatricula visible in the vitelline membrane therefore represents, 

 in the new-laid eg^, not the primary and simple germ, but the germ segmented 

 and ready to form the blastoderm. To see this properly, a fresh egg should be 

 opened in water ; the centre of gravity of the yolk being eccentric, the vitellus 

 passes in a certain diiection, bringing the cicatriculus to the highest pole of the 

 yolk, in the centre of the visible hemisphere. 



Accessory parts. — In passing through the oviduct, the ovum — reduced, as we 

 have seen, to the mass of the vitellus — is surrounded successively by : 1. A layer 

 of albiunen. 2. A testaceous membrane. 8. A shell. 



The aldumen — or " white " — forms three layers of different densities. As, in 

 its progression, the Qgg is submitted to a rotatory mo\'emeiit on its greater axis, 

 so the albumen undergoes a twist and a particular condensation along this axis ; 

 in this w\T,y are formed two kinds of coverings, that can be seen floating and 

 adhering to the yolk of eggs opened in water, and which are named the chalaza. 



The shell membrane {memhrana pufaminis), fibroid in appeai-ance, at the 

 thick pole is doubled into two layers to form a cavity — the air-c/taml/pr. This 

 cavity increases as development goes on, in order to meet the respiratory demands 

 of the embryo, or foetus ; while the yolk and albumen become progressively 

 expended as the foetus grows. 



The shell is an organic framework impregnated with lime salts ; but it is 



