DEFECTIVE PLUTEUS LARVAE. 379 



normal larvae if they formed normal gastrulae. This assumption 

 is quite unjustified, as my data on Arbacia show. 



PRELIMINARY EXPERIMENTS. 



Several years ago at the Marine Biological Laboratory I began 

 experiments to determine whether in the egg of Arbacia both 

 blastomeres isolated in the two cell stage would form normal 

 half size larvae. Each egg was handled separately, and the 

 development was followed for at least four days. The work of 

 the first season 1924 gave so few pairs of blastomeres each ot 

 which developed that the point could not be settled. At the 

 same time it became apparent that one half blastomeres give rise : 

 (i) to ciliated larvae which either do not gastrulate or do not form 

 a skeleton, (2) to larvae with incomplete or partial skeletons, and 

 (3) in a small number of cases to whole larvae of one half size. 

 These facts I reported briefly at the Washington Meeting ot the 

 Society of Zoologists in 1924. 



During the past two seasons at Woods Hole I have been able 

 to extend these observations on Arbacia, and during 1926 I made 

 a similar series on the egg of the sand dollar Echinarachnius . 

 The latter egg is much more favorable for such work, since it is 

 larger, has a membrane which is much less resistant, and w r hen 

 good lots of eggs are obtained, gives a high percentage of eggs 

 which develop into normal plutei. It is harder to get good lots 

 of eggs of Echinarachnius than of Arbacia at Woods Hole, and 

 therefore the total number of blastomeres followed is less. 



METHODS. 



The eggs were removed from the animals into sea water, 

 washed, and fertilized with motile sperm suspensions. During 

 the earlier work with Arbacia, the membranes were removed by 

 shaking a suspension of eggs in a small test tube for twenty 

 seconds at an interval of one to one and a half minutes after 

 fertilization. This violent treatment is necessary to break and 

 remove the very tough membrane of Arbacia, and it generally 

 breaks up or otherwise injures a large number of the eggs. It 

 was later found possible to remove the membranes with great 

 ease, either about fifteen minutes after fertilization or in the two 



