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 beneath 

 it 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 absence 

 of differentiation, but they agree in genesis and dis- 

 tribution, 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 Tunicata 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 forepart 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- 



