The Development of the Frog 5 

 The Egg 



Since every animal begins its individual ex- 

 istence as an ovum or egg, it may be well, 

 before taking up the study of the frog's ^g^, 

 to examine some &^^ that will more easily show 

 the different structures of a typical ovum, the 

 frog's ^'g^ being so large and so full of yolk 

 that it is difiicult for the beginner to distin- 

 guish its different parts. For this purpose the 

 eggs of the starfish, or of the sea-urchin, are 

 very convenient, and if some of these eggs be 

 properly stained and mounted, the main fea- 

 tures of their structure may be made out with- 

 out difficulty. 



The ovum, whether it be of microscopic 

 size or 30 mm. in diameter, as is the yolk of 

 the hen's ^gg, is always a single cell. Al- 

 though the &'gg of the common starfish is only 

 as large as a small grain of sand, yet if it 

 be examined under a moderate magnification 

 of the microscope, it will be found to be made 

 up of several distinct parts. Like most ova 

 it is spherical in shape, and is enclosed in a 

 thin cell-wall or vitelline membrane (Fig. 2). 

 In the granular, protoplasmic contents of the 

 6gg two regions may be distinguished : a 



