Nature of Formative Stimulation 139 



had entered into those eggs which develop after the addition 

 of star-fish sperm without subsequent treatment \\'ith a hyper- 

 tonic solution into normal larvae; while the eggs which behave 

 as if only an artificial membrane formation had taken place 

 do not contain any spermatozoon. 



This behavior of the eggs under the influence of foreign 

 sperm is comprehensible under the assumption that the 

 spermatozoon also causes the development of the egg through 

 two agencies; one of these agencies is a cytolytic substance, 

 a so-called lysin. This substance is probably situated at the 

 surface of the spermatozoon. This lysin only calls forth 

 the membrane formation and it acts like the but>Tic acid in 

 the method of artificial parthenogenesis. The second agency 

 seems to be more in the interior of the spermatozoon and it 

 exercises an influence similar to the short treatment of the egg 

 with a hypertonic solution. A normal development will result 

 only if the spermatozoon enters the egg since in this case only 

 both agencies, the cytolytic and the corrective, get into the 

 egg. We have already mentioned that foreign spermatozoa 

 penetrate only slowly into the egg. If a spermatozoon pene- 

 trates partially through the surface of the egg without entirely 

 penetrating into the protoplasm, enough of the lysin sticking 

 to the surface of the spermatozoon can be dissolved to cause 

 the cytolysis of the surface film of the egg which gives rise to the 

 membrane formation. Such eggs receive from the spermato- 

 zoon only the lysin, and they act therefore as if only the mem- 

 brane formation had been called forth in them by the treatment 

 with but>Tic acid, since in the formation of the membrane 

 the spermatozoon is thro\\'n out. 



In the eggs of Strongylocentrotus purpuratus the mem- 

 brane formation can in general only be called forth by living 

 star-fish sperm while the extract of dead star-fish sperm in the 

 same concentration remains without effect. This fact is of im- 

 portance to disprove the possibilit^'that the membrane formation 



