DEVELOPMENT OF THE EGG-CELL. 



139 



spondingly divided. At this stage the egg consists of 

 as many cells as there are nuclei in the tread. Hence, the 

 yellow ball of yelk of the impregnated egg, as it is laid, 

 and as we eat it every day, is already a many-celled body. 

 Its tread is composed of many cells, and is now dis- 

 tinguished as the germ-disc (discus blastodermicus). In 

 the eighth chapter we shall refer to this again. 



FIG. 12. A ripe egg-cell from the ovary 

 of a hen. The yellow nutritive yelk (c) is 

 composed of many concentric strata (<i) and 

 is surrounded by a thin yelk-membrane (a). 

 The cell-kernel, or germ-vesicle, lies in the 

 npper part, in the tread (6). From this the 

 white yelk passes into the centre of the 

 yelk-cavity (d'). The two kinds of yelk 

 are not, however, distinctly separated. 



After the ripe egg of the Bird (Fig. 12) has left the ovary 

 and has been fertilized in the oviduct, it surrounds itself 

 with various coverings which are secreted from the inner 

 surface of the oviduct. The thick layer of transparent 

 albumen first forms round the yellow yelk ; this is followed 

 by the formation of the outer calcareous shell, within which 

 lies another envelope of skin. All these coverings and 

 additions which are gradually formed around the egg, are of 

 no importance to the development of the embryo ; they are 

 parts that have nothing to do with the original simple egg- 

 cell. Even in the case of other animals we often find very 

 large eggs with thick coverings, for example, in that of 

 the Shark. In this case also the egg is originally exactly 

 similar to those of Mammals, that is, it is a simple naked 

 cell. But, as in the case of Birds, a considerable quantity 

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