252 THE DOCTRINE OF DESCENT. 
gastrula larva that the future dorsal side becomes flat- 
tened, and the protuberances arise, which shortly after 
close into the sheath of the spinal marrow, while under- 
neath originates this important cellular column, the 
chorda dorsalis, or notochord. With this the lancelet 
becomes a vertebrate animal, and the preceding phases 
do not (according to the view at one time inculcated 
by C. E. v. Baer respecting such phenomena) recall 
the inferior and undeveloped in general by the ab- 
sence of differentiation, but they agree in genesis and 
distribution, in the differentiation of their cellular layers, 
and in their totality, with the Gastrula phases of inver- 
tebrate animals. 
We are therefore fully justified in regarding these 
first incidents in the evolution of the Amphioxus as a 
reminiscence of the roots of the pedigree of the Verte- 
brata; and this direct indication of the descent of 
vertebrate from invertebrate animals is supported by a 
second and no less important discovery by the Russian 
naturalist. It is, that during their development a num- 
ber of the Testacea of the division of the Ascidians 
temporarily possess a spinal cord, and the rudiments 
of a vertebral column. Kowalewsky’s researches have. 
been ratified on all essential points and in many ways 
extended by Kupfer, and the facts which interest us 
may be explained by the diagram, Fig. 23, representing 
the point of the larva of an Ascidian in a somewhat 
advanced stage. The bulk of the Ascidian larva consists 
of a body of which our figure shows the whole, and a 
rudder-like tail. The appendages projecting from the 
body on the right are organs of adhesion, by means of 
which the larva fixes itself for its definitive transforma- 
