MICRODISSECTION STUDIES. 347 



immature eggs. This membrane becomes more pronounced as the 

 eggs reach their full growth and still more so as the egg matures. 

 In the sea-urchin the immature eggs exhibit no trace of a mem- 

 brane until the eggs begin maturation. In the mature unfertilized 

 sea-urchin egg the membrane has reached a development com- 

 parable to that of the half -grown immature egg of the starfish. 



6. The egg membrane rises off the surface of the egg upon fer- 

 tilization and constitutes the fertilization membrane. No appreci- 

 able diminution in volume of the egg occurs during this process. 



7. An egg, whose membrane has been removed, is fertilizable 

 and segments without a fertilization membrane. 



8. The hyaline plasma layer, which forms on the surface of the 

 sea-urchin and sand-dollar egg within ten minutes after fertiliza- 

 tion, binds the blastomeres together. In the starfish egg no such 

 layer is formed, and, if the fertilization membrane be removed, the 

 blastomeres tend to fall apart. 



9. The fertilizability and approach to normal development of 

 an egg fragment is directly proportional to the amount of a 

 substance which emanates from the germinal vesicle during 

 maturation. 



10. The unfertilized mature egg possesses a more solid cortex 

 of appreciable thickness inclosing a highly fluid interior. The 

 fluid interior of the starfish and sand-dollar eggs can be made to 

 ooze out through a tear in the cortex, whereupon it forms a sur- 

 face film on coming into contact with sea water. In this way the 

 internal and cortical material of the egg can be isolated from one 

 another. Both round up, the internal material immediately and 

 the cortical after some time. 



11. Endoplasmic material, possessing a small part of the original 

 cortex, is fertilizable and the approach to normal development is 

 in direct proportion to the amount of cortical material present. 

 The presence of even a small amount of cortical material causes 

 disintegrative changes to set in at about the same time as in a 

 whole egg. 



12. The following table gives, for the various kinds of frag- 

 ments of immature and mature starfish eggs, the length of time 

 that they withstand disintegration when left standing in seawater 

 and also whether they are or are not capable of being fertilized : 



