Ranidae 



surface of the egg seems to have spread quite to the edges of 

 this circle. The circle encloses a mass of white yolk, which pro- 

 trudes like a small cushion.^ (Fig. 206, A.) Throughout the 

 development, our knowledge will be limited to what we can see 

 on the outside. We can know very little, or nothing, about the 

 rapid and wonderful happenings within. However, certain 

 external changes will indicate internal conditions. This circle 

 with its protruding plug of yolk is the external sign of a splitting 

 or separation between cells, which is the beginning of the forma- 

 tion of the digestive tract. This digestive tract begins forming 

 at its posterior instead of at its head end, therefore this yolk- 

 plug marks a point very near the posterior end of the young 

 frog. 



Before the third day is over, the eggs have become elon- 

 gated and on many of them a visible groove is being formed 

 lengthwise along the top. (Fig. 206, B.) This groove marks 

 the back of the developing frog; so that now we know which 

 is to be the head and which the tail end, and which is the 

 right and which is the left side. 



By the fourth day, the eggs are greatly changed. The 

 egg has become still more elongated and the yolk-plug has wholly 

 retreated. The groove which began so simply has extended 

 along the whole length of the back (top of egg) and two great 

 folds of the surface are slowly rolling in over it, one on each 

 side. These folds are broader at the head end. (Fig. 207.) 

 This is perhaps the most interesting part of the development 

 that we shall see. This groove and the folds which are rolling 

 over it will form a tube within, which is the beginning of the 

 nervous system. The head end of the tube forms the brain 

 and the remainder makes the spinal cord. 



On the fifth day, so great has been the advancement that 

 we are again puzzled to account for appearances. (Fig. 208.) 

 The elongation is still greater, and head and tail ends are curved 

 somewhat to one side. The line of the back is nearly straight, 

 the yolk side (front) is very convex. The nerve-tube seems 

 to be wholly closed in, but the folds still persist to a certain ex- 

 tent over the head and along the back. The projection which 

 is to form the tail is very apparent. 



1 For technicalities concerning the fonnation of the blastopore and its subsequent history, 

 »ee Bibliography, pp. 241-250. 



178 



