THE DEVELOPMENT OF ALCYONIUM DIGITATUM. 
49' 
they increase slightly in size, and become somewhat paler in 
colour, and are thus distinct from newly-spawned eggs to the 
naked eye. 
They were pipetted into finger-bowls of “ outside 39 water 
(water brought into the laboratory from outside Plymouth 
Sound, and therefore in especially good condition), and 
development took place at the ordinary and by no means 
constant temperature of the laboratory. The developing 
eggs appeared to do equally well in outside, Berkefeld- 
filtered or ordinary tank water, and many larvae went through 
all the stages of development, settled and produced tentacles 
in the laboratory tank where the eggs were spawned. 
Twenty-four hours after spawning the embryos were all 
morulae, more or less advanced. 
Fresh colonies were continually added to the tank, and 
spent and unhealthy ones removed during the whole of the 
spawning season. Judging by the proportion of ripe ones 
brought in, this season reaches its height towards the end of 
January, and soon declines after that date. 
The segmenting egg (Text-fig. 1a) is in all stages typically 
spherical, though during segmentation some examples may 
become temporarily oval, regaining their globular shape later. 
At the close of the morula stage the embryo undergoes a 
curious change in shape. About the twenty to twenty-fourth 
hour the whole surface of the sphere is slowly drawn out into 
irregular blunt prominences with corresponding depressions 
(Text-fig*. 2), the component cells meantime undergoing a 
modification of structure and the segmentation cavity gradu- 
ally disappearing. This condition lasts for some time, but 
about the forty-fourth hour the knobs begin to withdraw 
again. Wilson (16) mentions a similar stage for Renilla and 
Leptogorgia, but apparently did not investigate it closely. 
As this definite stage is followed by the swimming planula,. 
it was convenient to call it the “ pre-planula ” stage. By the 
end of the third day of development the pre-planula no longer 
shows definite protuberances and depressions, these having 
softened down into a gently wrinkled outline (Text-fig. 3).. 
VOL. 62, PART 1. NEW SERIES. 4 
