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 egg, 

 to examine spme egg that will more easily show 

 the different structures of a typical ovum, the 

 frog's egg being so large and so full of yolk 

 that it is difficult 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 egg, is always a single cell. Al- 

 though the egg 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 

 egg two regions may be distinguished : a 



