ECHINODERMS 93 



are sectioned vertically, there are slight differences in development accord- 

 ing to the plane of the section. These can be best interpreted by the hypo- 

 thesis that one side already has a slight tendency to become ventral, and 

 usually does so. If a ventral half is separated from a dorsal half, the ventral 

 face appears in its original position in the former, but in tlie dorsal half, 

 the axis may be reversed, so that the eventual ventral side of this half- 

 embryo appears on the side which, in the undisturbed egg, would have 

 been dorsal. 



This original organisation is only a labile one, and can be overcome by a 

 number of different influences. It becomes determined at about eight 

 hours after fertilisation. If eggs are strongly stretched before this, by 

 being sucked into a tube with a narrow lumen, the dorso-ventral axis is 

 altered so as to run along the length of the elongated egg. Strong staining 

 of one end of such an egg with Nile Blue sulphate will cause this end to 

 become dorsal; perhaps it would be better to say that it will cause the 

 other end to become ventral, since it is probable that the ventral side plays 

 the lead in the development of the axis, and that it is a suppressive action 

 of over-staining which is the operative mechanism in the experiment. 

 Many other chemical substances can influence the determination of the 

 axis (see Review in Lehmann 1945). Gustafson and Savhagen (1949) have 

 recently shown that weak solutions of detergents will suppress the develop- 

 ment of the oral or ventral side entirely, so that radially symmetrical 

 larvae are formed. A similar result was produced by Horstadius and 

 Gustafson (1954) who treated the eggs shortly after fertilisation with 

 antagonistic analogues (or 'anti-metabolites') of certain vitamins, etc. 

 They suggested indeed that these substances were actually operating as 

 detergents rather than by inhibiting the growth-promoting properties of 

 the vitamins. 



SUGGESTED READING 



Horstadius 1939, 1949, Gustafson and Lenique 1952 or Gustafson 1953, I954, Hultin 

 1953^- 



