328 Biology in America 



of animals are practically unknown in nature, and yet Loeb 

 has succeeded in cross fertilizing the sea urchin 's egg with the 

 sperm of several species of starfish and one of the brittle 

 stars, by simply adding a little sodium hydroxide or car- 

 bonate to the sea water containing the eggs. 



The entrance of the sperm into the egg induces changes 

 in the latter which can likewise be induced by chemical 

 means. When the sperm of a sea urchin strikes the egg the 

 two adhere to each other, due probably to a sticky secretion 

 of the latter. A few moments later the very delicate mem- 

 brane surrounding the egg is pushed off from the surface 

 and considerably thickened, due probably to absorption of 

 water. The cause of this membrane formation (or better, 

 membrane extrusion) is the liquefying of the surface of the 

 egg just beneath the membrane and its consequent absorp- 

 tion of water. Subsequent to this membrane formation the 

 sperm head or nucleus penetrates still farther into the egg 

 leaving the tail adherent to the egg membrane, while the egg 

 nucleus advances to meet it, the two fuse and fertilization^ is 

 accomplished, to be followed shortly by the division of the 

 egg into first two, then four, eight, sixteen cells, and so on. 

 Many workers have succeeded in imitating the processes of 

 fertilization and causing the eggs of a large number of spe- 

 cies of animals to develop parthenogenetically by various' 

 methods of treatment. In the case of the sea urchin Loeb 

 first treats the egg with some chemical (i.e., butyric or other 

 monobasic fatty acid) which induces membrane formation, 

 and then follows this treatment by placing the egg in sea 

 water containing a little more salt than usual, or into nor- 

 mal sea water lacking oxygen. The two procedures are es- 

 sential to development, for if the first alone be employed the 

 egg disintegrates after extruding its membrane, without fur- 

 ther development. A similar result sometimes occurs when 

 a sea urchin egg is fertilized by starfish sperm. Here the 

 entrance of the sperm is very slow, some ten to fifty min- 

 utes compared with about a minute in the case of sperm of 

 the same species. In the former case, owing no doubt to the* 

 slow penetration of the sperm, the latter does not always en- 

 ter the egg, but remains attached to the extruded membrane. 

 It seems therefore that the sperm secretes two distinct sub- 

 stances, one of which causes liquefaction of the surface layer 

 of the egg, with consequent absorption of water and extru- 

 sion of the membrane, while the other causes the initial de- 

 velopment (division of the egg) to ensue. The action of this 

 second substance is not yet clearly understood but apart 

 from the experiments in artificial parthenogenesis and the 

 occasional cessation of development after membrane forma- 



