MICRODISSECTION STUDIES. 



333 



containing a few eggs on a gelatin-coated slide. The eggs were 

 rolled over by means of a micro-needle and only those which main- 

 tained their spherical shape were measured. \\"\ih a micro-pipette 

 sperm were introduced into the drop without disturbing the rela- 

 tive positions of the eggs. A second method was to place several 

 eggs in a hanging drop in a Barber moist chamber. . By piercing 

 the surrounding jelly with a needle the egg to be measured could 

 be held suspended in the middle of the drop. Numerous measure- 

 ments of the starfish egg were made at different times through 

 several summers and in every case the egg maintained its original 

 size as the fertilization membrane rose off its surface. Not only 

 does the egg not decrease in volume, but it slightly increases in 

 size until segmentation occurs. The accompanying table is one 

 sample of the measurements made: 



The conclusions from this table apply both to starfish and sea- 

 urchin eggs. They may not necessarily be true for other species. 



Fig. 17 shows successive steps in pulling a starfish egg out of 

 its fertilization membrane. No second membrane is ever formed 

 even with superimposed insemination. Occasionally the hyaline 

 plasma layer in such an extruded egg swells up and simulates a 

 second membrane, and it is probably this that has been described 

 by certain investigators as a second fertilization membrane. The 

 hyaline plasma layer will be discussed under heading 5. 



An unfertilized mature sea-urchin egg may be rolled about and 

 its contents churned to the extent of producing " fountain cur- 

 rents " within the egg (Chambers, 'ij-b). This is done by push- 

 ing an egg in a drop shallow enough to compress the egg. Cur- 

 rents are produced which flow backward immediately under the 

 surface of the egg and forward along its central axis (Fig. 18). 

 By careful manipulation it is possible to do this without rupturing 



