The Injiuence of External Factors on Development 177 



after penetrating the membrane. If such should occur, the con- 

 centration of the sugar solution within the membrane would be 

 higher than that without and this might produce a strong en- 

 dosmosis which would result in the swelling of the yolk. The 

 fact that the yolk or egg does not swell in fresh water or in hypo- 

 tonic solutions has been used as an argument to show that they are 

 immune to osmotic effects. That they are not, however, entirely 

 immune to such effects has been shown above. 



THE SEA-WATER SOLUTIONS OF SUGAR 



The yolks shrink in these solutions in the manner mentioned 

 above, often within one hour after having been put into the solu- 

 tion. The shrinkage of the yolk occurs so promptly that the outer 

 membrane fits loosely around the egg, and often shows an indenta- 

 tion on one side. There occurs below the blastoderm in many 

 cases a bubble-like appearance which disappears, however, as de- 

 velopment progresses. When about forty hours old many eggs die 

 in the 1.33 m solution, which exerts a pressure of about twenty-six 

 atmospheres more than ordinary sea-water. The dead eggs usually 

 have a polar ball of protoplasm on the shrunken yolk. The living 

 embryos have the tail end indistinctly indicated and the blasto- 

 pore remains open much longer than is usually the case. If eggs 

 were removed from these solutions and put into sea-water at any 

 time before twenty hours they soon recovered and developed nor- 

 mally with the exception of those from the stronger solutions, in 

 which case the yolk rarely recovered its full size, and in consequence 

 the pericardium seemed abnormally large. As development con- 

 tmues in the stronger solutions, the yolks become smaller and 

 smaller, and the embryos are likewise much dwarfed with very 

 weak heart contractions which begin only some time after the con- 

 trol embryos have etsablished a free circulation. Fig. 3 shows 

 many of these characters in an embryo when five days old in a mole- 

 cular solution of sugar in sea-water. Fig. 4 shows the condition of 

 a control embryo of the same age. The body of the embryo is 

 often abnormally bent or twisted on the yolk. The sluggish circu- 

 lation at times allows the blood to accumulate in certain vessels, 

 commonly in the veins along the ventral line of the tail, as large 



