THE BIOLOGY OF THE CELL SURFACE 



convinces me that it resembles those of three species of 

 Asterias which I have studied, and that its classification as 

 a normally parthenogenetic egg is unwarranted. 



Eggs of the marine worm, Chaetopterns, like those of the 

 starfish fertilizable in the stage of first maturation, when 

 treated with hypertonic sea-water develop into bizarre and 

 wholly abnormal swimming forms through repeated nuclear 

 divisions in the cytoplasm which fails to divide, a condition 

 known as differentiation without cleavage.^ Following 

 exposure to hypertonic sea-water, other eggs fertilizable 

 in the stage of first maturation, e.g., those of Podarke'^ and 

 of Amphitrite,^ both worms, develop without cytoplasmic 

 cleavage. For the egg of Chactopterus and for that of the 

 small marine clam, Cumingia, increased temperature of 

 the sea-water, induces the best type of parthenogenetic 

 development — with cytoplasmic as well as nuclear divisions. 



The Q^g of Chaetopterus merits attention because Mead 

 used it in his interesting experiments. Previously in 

 his paper on the normal fertilization-process in this egg 

 he had set forth a strong argument against the theory 

 that fertilization is due to the introduction of centrosomes 

 into the egg by the spermatozoon. His work on the effect 

 of salt solutions in initiating development was the logical 

 outcome of his main conclusion in the work on fertilization, 

 a later statement of which reveals his clear conception of the 

 initiation of development as a chemical process. His work 

 on inducing parthenogenesis, therefore, was no mere acci- 

 dental experiment; rather, at its basis was a well defined 

 working hypothesis. His was the first work based on the 

 assumption that development can be experimentally 

 induced. Why Mead so suddenly dropped this promising 

 line of research is strange. It was justly said by Loeb, 



^ Lillie, /po6. 



~ Treadwell, igo2. 



^ Scott, igo6. 



216 



