DEVELOPMENT IN ARBACIA. 153 



dilutions but without the brief tore-treatment with hypotonic 

 sea-water. Results whieh harmonize with these but prove less 

 satisfactory on account of injuries to the eggs can be gotten by 

 the use of heat. In this case one might think of a parthenogenetic 

 ef?ect, but in Arhacia at least, it is not easy to confuse the usual 

 parthenogenetic cleavage with normal two or four-cell stages. 



It is very easy to misunderstand these experiments and to 

 draw wrong conclusions. There is no more doubt m Arhacia 

 pnnctulata than in any other form that a single spermatozoon is 

 Efficient to carry out the biparental effect. Furthermore the 

 experiments with dilute sperm do not in anyway enable us to 

 prejudge what would happen in another egg under s.m.lar con- 

 ditions nor do they warrant the inference that the imt.ation of 

 development by a single sperm is impossible m Arbacm ova 

 deprived of their superficial coverings. I feel very sure of this 

 however: In Arbacia the appearance of the fertilization membrane 

 after insemination is a sign that the egg investments have allowed 

 the sperm to pass through. This passage has been possible 

 because the coverings have changed. The change depends on a 

 synchronous softening and absorption of water, the latter having 

 consequences as the result of which the membrane becomes vis- 

 ible. Inasmuch as the becoming visible of the membrane is a 

 reliable index of fertilization, and one of the consequences of 

 fertilization is the division of the ovum, we may say that the 

 initiation of development by a single spermatozoon in this case 

 is impossible because a single sperm cannot effect those changes m 

 the egg-coverings which will permit it to reach the protoplasmic 

 surface film that lies beneath. The situation is exactly as though 

 the entrance to a room were blocked by a barrier which a smgle 

 man could not break down, although a group of ten might. 

 Once broken down, any one of the men could cross the threshold, 

 but for the opportunity of doing this, the services of the others 

 would be needed. With this analogy in mind, the statemen 

 that a single spermatozoon cannot except possibly under special 

 conditions, fertilize the normally invested egg of Arba^m punctu- 

 lata, would appear to agree with the facts. 



Zoological Laboratory. 



University of Michigan, ■ 

 November 12, 1914. 



