212 Education through Nature 



Different Kinds of Eggs. The development of the 

 eggs of all higher animals resembles that of the frog 

 more or less completely, according as the egg con- 

 tains a large or a small quantity of yolk. The latter 

 is food material stored up in the egg. Yolk varies 

 greatly in amount in the eggs of different animals, 

 being the chief cause of difference in size of different 

 eggs. Most eggs are spherical or oval. The eggs of 

 some insects are greatly elongated or cylindrical, as 

 in flies and grasshoppers. In the latter case the 

 polarity which appears in the embryo is foreshadowed 

 in the undeveloped egg. 



Accessory parts exist in many eggs, that are often 

 responsible both for the extraordinary size and the 

 unusual shape of some eggs. In the frog's egg, the 

 gelatinous mass surrounding it, is a secretion of the 

 oviduct, a tube conveying the egg from the ovary, 

 where it is produced. This gelatinous substance 

 corresponds to the albumen or white of birds' eggs, 

 which is also a secretion of the oviduct. In this case, 

 however, it becomes surrounded, later, by a new 

 secretion of the lower end of the oviduct, which hardens 

 into a shell. The shell of the hen's egg does not repre- 

 sent the original cell- wall or membrane, as in the frog's 

 egg, but is an entirely foreign coat added to the egg 

 as it passes down the oviduct. . 



Both the albumen and the yolk of these eggs are 

 secretions intended for nourishment of the growing 

 embryo. Eggs having but little yolk, as those of 

 mammals, remain attached to the parent in a special 

 organ, the uterus. By this means the deficiency in 

 yolk material is made good by nourishment derived 

 directly from the blood of the mother, and after birth 

 by the mother's milk. The milk glands in mammals 

 may, therefore, be compared, functionally, to the 

 glands in the oviduct of the frog or the hen which 

 fcecrete the albumen- 



