DEVELOPMENT OF SPELERPES BILINEATUS 189 



The objection may be raised that the preserved material does 

 not represent actual conditions in the living egg, but there are 

 several arguments against such a point of view. In the first place, 

 it is well known that it is very difficult to separate amphibian 

 blastomeres. They tend to disintegrate after separation, which 

 indicates some sort of continuity. In the second place, material 

 fixed in some of the picric mixtures, which have a tendency to 

 separate the blastomeres, show the same phenomena. The cell- 

 walls when present are perfectly distinct except in a zone of 

 transition leading to the region where the egg material is still 

 unseparated. Third, there is no evidence that the cell-wall had 

 previously existed and been torn from its place in sectioning. 

 The contents of one cell simply pass without any break or inter- 

 ruption of any sort into the next. 



Changes in the size of eggs 



Often when a group of eggs is examined as they lie upon the stone 

 to which they are attached, it is noticed at once that some of the 

 eggs are considerably larger than others. Close examination 

 shows that the largest eggs have a translucent upper hemisphere, 

 indicating an advanced stage of development. Besides this, eggs 

 in the same cleavage stages often differ much in size. Observa- 

 tions made on a very few eggs indicate, first, that the relative 

 size differences are maintained throughout the early course of 

 development, and, second, that after the first day, there is a 

 gradual increase in the size of the eggs until after the close of 

 gastrulation, when a slight decrease occurs. An extended series 

 of observations was not made, the object of this series being to 

 determine somewhat more accurately than bj^ mere inspection, 

 the time and amounts of increase in size of the eggs. 



Four eggs were taken, two 'large,' and two 'small.' Careful 

 camera drawings of the outlines of the eggs were made at intervals, 

 usually about a day apart.' Since the outlines as drawn by the 

 camera are oval, the measurements in the following table were 

 made on the shorter axis. Variations in focusing have made 



THE AMERICAN JOURNAL OP ANATOMY, VOL. 12, NO. 2 



