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tlius followed step by step. But contrast with this the 
conditions offered by the development of Elasmobranchs, 
Reptiles and Mammals ! Here an adult animal has to be 
sacrificed to gain a very small number of eggs; and a long 
series of years is requisite before anything like a continuous 
series of stages can be obtained. A great many of the 
difficulties and disputes which have arisen in connection with 
the interpretation of the early developmental stages of the 
higher Vertebi’ata are due to this cause alone. Again, if the 
theory of evolution be true at all, the developmental stages 
of the lower Vertebrata should be less modified than those of 
the higher, and should contain the key to them. To quote 
the ‘Text-book of Embryology’ of the late Professor F. M. 
Balfour on the subject of the development of the fowl, “the 
subject itself is by no means commensurate with the atten- 
tion it has received. The characters which belong to the 
formation of the layers in Sauropsida are secondarily derived 
from those in Icthyopsida, and are of but little importance for 
the general questions which concern the nature and origin of 
the germinal layers.” It is therefore amazing to find 
Hubrecht advising students of embryology to tackle Amphi- 
bia rather than Amphioxus, and mammals rather than 
cartilaginous fishes. The embryology of the higher forms 
may show how the process of the formation of germinal layers 
may be modified, but not how it originated. 
Under these circumstances we may first turn to the 
development of the simple Ascidians. The figures given by 
van Beneden and Julin for Clavellina and by Kowalevsky for 
Ascidia, as reproduced by Korschelt and Heider, are striking-ly 
like the figures I have given above for the gastrulation of 
Amphioxus. We see the advance of the dorsal anterior lip 
of the blastopore, and the coincident formation of the neural 
plate above it, and then in the second stage the up-gi’owth of 
the lateral and ventral lips and consequent narrowing of the 
blastopore. The main differences are ; (1) the contrast between 
endoderm and ectoderm cells is greater than in Am phioxus, 
and the whole number of cells in both layers is smaller and the 
