EXPERIMENTS WITH FROGS EGGS. 75 



shown in Fig. 11, and another at the beginning of gastrulation in 

 Fig. 12. Both show a large segmentation cavity. The results 

 may be summed up as follows : If the egg in the two- or four 

 cell-stage is taken from the water and kept from drying, it will 

 develop, and the segmentation cavity will be formed, which, 

 although sometimes smaller than the normal, if the egg is very 

 dry, yet its presence under these adverse conditions shows that 

 it must be due to a secretion poured out by the blastula cells, and 

 that it is not due directly to the passage of the water from the 

 outside into the egg. 



I have found that the eggs of Fundtilus lictcroclitns will also 

 develop out of water if simply placed on a glass plate in a moist 

 atmosphere. The eggs of the starfish will pass through several 

 segmentation stages under similar conditions, but so little water 

 remains on the glass that it evaporates quickly and the eggs are 

 so delicate that they cannot withstand the drying. The blastula 

 becomes flattened and is nearly solid, but on being placed again 

 in sea water it quickly rounds up, and the segmentation cavity 

 appears. The normal segmentation cavity in the starfish is very 

 large and is early formed. Its absence in the eggs out of water 

 may be due to the flattening of the egg, but possibly its absence 

 is due to the necessity of the cells to absorb water in order that 

 it may be formed. Those who have studied the segmentation 

 of the sea urchin egg in water under compression, as when it 

 is placed between compressing plates, have observed the absence 

 of the segmentation cavity, even when the segmenting egg has 

 become two-layered. In the frog also, as we have seen, the 

 compression may be responsible for the suppression of the full 

 development of the blastocoel, but since the compression is not 

 carried so far in the latter case, the formation of the cavity is not 

 suppressed, and the fact that it forms at all under these adverse 

 conditions goes to show that its origin is due to the activity of 

 the surrounding cells 



THE INCREASE IN THE SIZE OF THE EGG DURING THE SEGMEN- 

 TATION STAGES AND THE INCREASE OF THE BLASTOCCEL. 



I measured some eggs of Rana sylvcstris at different stages of 

 development in order to see whether the increase in size of the 



