72 MCMURRICH. [Vol. XI. 



What all the conditions may be which determine the forma- 

 tion of the vitelline membrane is at present obscure, but the 

 primary condition is understood more especially through the 

 observations of Fol and the Hertwigs ('87) to be normally 

 a stimulus imparted to the egg protoplasm by the spermato- 

 zoon. Herbst {'93) has shown that it is possible to imitate 

 this natural stimulus and call forth the membrane in unferti- 

 lized eggs by subjecting them to the action of various chemical 

 substances such as Benzol, Toluol, Xylol, and others ; and 

 the Hertwigs found that exposure to the action of certain 

 poisons such as cocain and chinium sulphate deprived the pro- 

 toplasm to a greater or less extent of the power of responding 

 to the normal stimulus. The membrane appears to be the 

 outer layer of the protoplasm of the ovum, hardened or thrown 

 off as a result of a stimulation of the protoplasm. Such a 

 cause for its formation explains its relations to the polar 

 globules of Jaera. It is entirely unrelated to their formation, 

 as their varying position within or without the vitelline mem- 

 brane demonstrates, though it is possible that their formation, 

 that is to say, nuclear division, may also be more or less 

 influenced by the penetration of the spermatozoon. Why the 

 membrane does not form in Honiarus and other forms it is 

 difficult to say ; it can hardly be a case of inhibition of the 

 stimulus due to the pressure of a considerable amount of yolk, 

 since the ova of Crangon have a less abundant yolk than those 

 of Porcellio, yet the former do not develop a vitelline mem- 

 brane while the latter do. Still it is possible that the yolk 

 may have some effect, since it seemed that in Porcellio the 

 formation of the membrane was in some cases very much 

 belated, not appearing until after segmentation had begun ; 

 opportunities for making conclusive observations on Porcellio 

 were, however, not afforded me. 



