THE FORMATION OP THE LAYERS IN AMPHIOXUS. 313 
of the layers, seeing that it is precisely in i*espect to this 
process that the modifications induced by the adaptation of 
the stock to a burrowing life do not make themselves felt. 
The egg of Amphioxus is relatively free from the factor 
that produces the greatest amount of disturbance in the 
development of the eggs of the higher Vertebrata, viz. food- 
yolk. Moreover, it has an extremely short embryonic life 
within the shelter of the egg-shell and a long larval exist- 
ence ; and a long larval existence is everywhere a primitive 
condition of affairs. As Korschelt and Heider in their 
‘ Text-book of Embryology ’ judicially remark, reproduction 
by ciliated larvas cannot be interpreted otherwise than as a 
primitive feature. But the best proof of all, that the develop- 
ment of Amphioxus is really primitive, is to be found in the 
way it lends itself to the harmonious explanation of the 
development of the higher Vertebrata, assuming as a modify- 
ing factor only the disturbing influence of food-yolk, as I shall 
now endeavour to set forth. 
In our search for other types of development amongst 
Vertebrata which may most readily be compared with that of 
Am])hioxus, there is one most important consideration to be 
borne in mind, viz. that a very large amount of material of 
the earliest stnges of development is needed for a successful 
analysis of the process of formation of the layers. The closer 
to one another are the successive stages available to the 
investigator, the less likelihood of error will there be in his 
results. Such a condition of affairs is possible in the case of 
very few types of Vertebrata. In reviewing the list of 
Vertebrata which have been studied, we see that this condition 
is fulfilled only in the case of Ascidians, of Urodela and 
Anura amongst Amphibia, and to a lesser extent of 
Cyclostomata, Dipnoi, Ganoidei and Teleostei amongst 
fishes, but these piscine groups have not been studied with 
anything like the care that has been lavished on Ascidians 
and Amphibia. 
In these two last-named gronps thousands of eggs of every 
assignable stage can be easily obtained and the processes 
