8 THE POPULAR SCIENCE MONTHLY 



If no spermatozoon enters into it, it perishes after a comparatively 

 short time, in some animals in a few hours, in others in a few days or 

 weeks. If, however, a spermatozoon enters into the egg, the latter 

 begins to develop, i. e., the nucleus begins to divide into two nuclei and 

 the egg which heretofore consisted of one cell is divided into two cells. 

 Subsequently each nucleus and each cell divides again into two, and 

 so on. These cells have in many eggs the tendency to remain at the 

 surface of the egg or to creep to the surface and later such an egg forms 

 a hollow sphere whose shell consists of a large number of cells. On 

 the outer surface of this hollow sphere cilia are formed and the egg 

 is now transformed into a free-swimming larva. Then an intestine 

 develops through the growing in of cells in one region of the blastula 

 and gradually the other organs, skeleton, vascular system, etc., origi- 

 nate. Embryologists had noticed that occasionally the unfertilized 

 eggs of certain animals, e. g., sea-urchins, worms, or even birds, show 

 a tendency to a nuclear or even a cell division; and R. Hertwig, Mead 

 and Morgan had succeeded in inducing one or more cell divisions 

 artificially in such eggs. But the cell divisions in these cases never 

 led to the development of a larva, but at the best to the formation of 

 an abnormal mass of cells which soon perished. 



I succeeded twelve years ago in causing the unfertilized eggs of the 

 sea-urchin to develop into swimming larvae by treating them with sea- 

 water, the concentration of which was raised through the addition of a 

 small but definite quantity of a salt or sugar. The eggs were put for 

 two hours into a solution the osmotic pressure of which had been raised 

 to a certain height. When the eggs were put back into normal sea- 

 water they developed into larvae and a part of these larvae formed an 

 intestine and a skeleton. The same result was obtained in the eggs 

 of other animals, starfish, worms and mollusks. These experiments 

 proved the possibility of substituting physico-chemical agencies for the 

 action of the living spermatozoon, but did not yet explain how the 

 spermatozoon causes the development of the egg, since in these experi- 

 ments the action of the spermatozoon upon the egg was very incom- 

 pletely imitated. When a spermatozoon enters into the egg it causes 

 primarily a change in the surface of the egg which results in the 

 formation of the so-called membrane of fertilization. This phenom- 

 enon of membrane formation which had always been considered as a 

 phenomenon of minor importance did not occur in my original method 

 of treating the egg with hypertonic sea-water. Six years ago while 

 experimenting on the Calif ornian sea-urchin, Strongylocentrotus pur- 

 puratus, I succeeded in finding a method of causing the unfertilized 

 egg to form a membrane without injuring the egg. This method con- 

 sists in treating the eggs for from one to two minutes with sea-water 



