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room when they were about 3 mm locg. About the tenth day after 

 this stage the balancers appear decidedly more transparent than pre- 

 viously. The transparency increases and often a small amount of 

 brick red pigment is visible in the distal end. About the fourteenth 

 day after the 3 mm stage the balancers drop ofl"^). 



Fig. 3 shows a transverse section through the body (longitudinal 

 through the balancer) at the 6 mm stage. It consists of an evagination of 

 the ectoderm into which the mesoderm is growing. The heavy black 



Fig. 3. Longitudinal section through right balancer (transverse through body of 

 embryo) of a 6 mm embryo, b. m basement membrane, ec ectoderm, en entoderm. 

 mes mesoderm. ^ 440. 



Hne (&. m) represents the first appearance of the basement membrane. 

 It is not present on the caudal third of the sucker. It lies partly 



1) Prior to the 3 mm stage the eggs were kept in water at 5*^ C; 

 in the Laboratory the temperature ranged from 15 ** C to 25 "^ C. The 

 embryos develop much more rapidly at the higher temperature. 



