The writer suggests the following explanation of the phenomenon. The egg sea water contains a substance which forms a precipitate with a substance on the surface of the spermatozoön whereby the latter becomes slightly sticky. This precipitate is slowly soluble in sea water and the more rapidly the more alkaline (within certain limits). Only when the spermatozoa run against each other with a certain impact will they stick together, as Lillie suggested. Lillie assumes that this agglutinating substance contained in egg sea water is required to bring about fertilization and he therefore calls it “fertilizin.”[72] But this assumption seems to go beyond the facts inasmuch as the existence of such an agglutinating substance can only be proved in a few species of animals (sea urchins and annelids); and as, moreover, sea-urchin sperm can fertilize eggs which will not cause the sperm to agglutinate, _e. g._, the egg of _franciscanus_ can be fertilized by sperm of _purpuratus_, although the egg sea water of _franciscanus_ causes no agglutination of the sperm of _purpuratus_. When the jelly surrounding the egg of the Californian sea urchin _S. purpuratus_ is dissolved with acid and the eggs are washed, the eggs will not cause any more sperm agglutination; and yet one hundred per cent. of such eggs can be fertilized by sperm.[73] [72] Lillie, F. R., _loc. cit._ [73] Loeb, J., _Jour. Exper. Zoöl_., 1914, xvii., 123; _Am. Naturalist_, 1915, xlix., 257. 5. It is well known that if an egg is once fertilized it becomes impermeable for other spermatozoa. This cannot well be due to the fact that the egg develops; for the writer found some time ago that eggs of _Strongylocentrotus purpuratus_ which are induced to develop by means of artificial parthenogenesis can be fertilized by sperm. The following observation leaves no doubts in this respect. When the unfertilized eggs of _purpuratus_ are put for two hours into hypertonic sea water (50 c.c. of sea water+8 c.c. 2-1/2 m NaCl) and then transferred into sea water it occasionally happens that a certain percentage of the eggs will begin to divide into 2, 4, 8 or more cells, without developing any further. When to such eggs after they have remained in the resting stage for a number of hours or a day, sperm is added, some or all of the blastomeres form a fertilization membrane and now begin to develop into larvæ; and if the spermatozoön gets into a blastomere of the 2- or 4-cell stage normal plutei will result. When the sperm is added while the eggs are in active parthenogenetic cell division the individual blastomeres into which a spermatozoön enters will also form a fertilization membrane, but such blastomeres perish very rapidly. It is not yet possible to state why it should make such a difference for the possibility of development whether the spermatozoön enters into a blastomere when at rest or when it is in active nuclear division, although the idea presents itself that in the latter case an abnormal