448 



SCIENCE 



[N. S. Vol. XXXVIII. No. 



the egg divides, some of these strands are 

 drawn down between the cleavage cells, and 

 as certain portions of the surface of these are 

 further removed from the fertilization mem- 

 brane than the original egg, the inner limit of 

 this membrane, as well as the perivitelline 

 space itself, becomes visible. The perivitelline 

 space seems to be identical with the so-called 

 " hyaline plasma-layer," and homologous with 

 the perivitelline space of the fertilized starfish 

 egg. 



2. By means of hypertonic solutions as well 

 as by extract of themselves, sea-urchin eggs 

 can be induced to divide without the appear- 

 ance of a fertilization membrane. Develop- 

 ment, however, does not proceed normally be- 

 cause the blastomeres fall apart. Since the 

 vitelline membrane is tightly glued to the 

 surface of this egg and a perivitelline space 

 appears after the membrane has swollen, 

 eggs dividing without the formation of this 

 space have the membrane adhering to the 

 resulting blastomeres. In consequence, these 

 cells, instead of being in intimate contact 

 with one another as they normally are, are 

 each enclosed in a separate vitelline mem- 

 brane. In other words, when the vitelline 

 membrane is not lifted off the egg surface, it 

 divides with the egg, which is what one would 

 expect. If this idea is correct, cleavage cells 

 which have originated by the division of an 

 egg without a " fertilization " membrane 

 should be able to " form " such membranes 

 under suitable conditions, and this I have 

 observed. Immersed in dilute sea water, iso- 

 lated cleavage cells, derived from ova which 

 have not formed " fertilization " membranes, 

 form them in from one to two minutes. 



3. Egg fragments can also be produced by 

 shaking.'. No fertilization membranes appear 

 in such eggs or their fragments as the result 

 of the mechanical agitation, but when treated 

 with dilute sea water or sperm, membranes 

 appear in some of the fragments, but not in 

 others. Both kinds of fragments have been 

 fertilized with sperm and allowed to develop, 

 some with and some without the membrane. 

 This result can only be understood if we ac- 

 cept Kite's discovery that the fertilization 

 membrane in Arhacia punctulata appears 



when a preexisting jelly, closely adherent to 

 the surface of the egg, swells and changes its 

 optical properties. 



4. From the above experiments one may in- 

 fer that a fertilization membrane may appear 

 around part of an egg, instead of the whole. 

 If Kite's jelly is ruptured the egg flows par- 

 tially through the hole in the membrane, and 

 assumes a dumbbell shape. If it is now fertil- 

 ized with sperm, or treated with dilute sea 

 water, a fertilization membrane appears on 

 one sphere of the dumbbell, but not on the 

 other. Such eggs are capable of develop- 

 ment. 



5. The appearance of a fertilization mem- 

 brane in Arhacia punctulata is not a function 

 of the living egg, for if the egg is crushed, or 

 even dried completely in a desiccator for days, 

 membranes still appear after proper treat- 

 ment. 



V. WHAT MAKES THE FERTILIZATION MEMBRANE 

 APPEAR NORMALLY? 



If the interpretation given to the results 

 outlined is correct for Arhacia punctulata, it 

 is easy to see why the fertilization membrane 

 should appear in dilute sea water, or in dis- 

 tilled water. But why does it appear under 

 normal conditions in sea water? 



The exact mechanism of the process is not 

 yet clear, but it seems to be a function of the 

 number of sperm present. If one insem- 

 inates eggs very carefully so that not more 

 than four or five spermatozoa come into con- 

 tact with each one, the fertilization membrane 

 does not appear. I have repeated this experi- 

 ment many times and have controlled it by 

 the most careful observations with different 

 powers on fresh material as well as stained. 

 Such preparations show sperm plainly ad- 

 hering to Kite's jelly in every egg, but the 

 " membrane " does not appear. Eggs treated 

 in this manner do not develop, although some 

 of the smaller ones may form asters. What it 

 is in the sperm that brings about the swelling 

 of the jelly has not yet been determined. 

 However, beautiful fertilization membranes 

 may be caused to appear in two to three hours 

 by treating the eggs with minute infusoria. 



