332 ROBERT CHAMBERS. 



found continuous with the fertilization membrane. The protrusion 

 subsequently pinched itself off and persisted in a sac-like protuber- 

 ance of the fertilization membrane (Fig. i6-d-c). 



In all of the various eggs studied a change in the consistency 

 of the membrane takes place very soon after it has been elevated. 

 The membrane, at first very soft and delicate, progressively 

 toughens until it becomes almost parchment-like during the later 

 segmentation stages. It is of interest to note that Harvey ('10) 

 found a difference between the unfertilized and the fertilized 

 sea-urchin egg when subjected to sulfuric acid. The acid dis- 

 solves the unfertilized egg completely, whereas it dissolves all 

 of the fertilized egg except the fertilization membrane. Some 

 chemical change apparently takes place as the membrane lifts 

 off the egg. 



Outside the membrane is a considerable zone of a structureless 

 jelly. In the sand-dollar egg the jelly very loosely adheres to the 

 membrane. On cutting into the jelly the egg with its membrane 

 easily slips out. This is to a somewhat lesser degree true for 

 the starfish egg. In the starfish egg one often, sees the under sur- 

 face of the jelly pushed away from the surface of the unfertilized 

 egg by the protruding polar body. 



The question as to whether the membrane lifts off the surface of 

 the egg or whether the egg shrinks leaving the membrane behind 

 has been raised by Glaser ('14) in spite of McClendon's ('10) 

 statement to the contrary. Glaser, by making a large series of 

 measurements, claims that the egg shrinks upon fertilization, and 

 that the initial diameter of the completed fertilization membrane is 

 equal to that of the unfertilized egg. Glaser's measurements were 

 made on the assumption that the eggs always maintain a spherical 

 shape. This is not true. The mature unfertilized egg is very soft 

 and if allowed to lie on the bottom of a glass dish tends to flatten 

 into the shape of a disc. Upon fertilization the egg rounds up as 

 the fertilization membrane leaves its surface. One can readily see 

 if the observations are taken of eggs in one plane only that erro- 

 neous conclusions may be arrived at. 



I used two methods to ascertain the diameter of starfish eggs 

 before and after fertilization. One method was to place a drop 



