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 differ- 

 entiation, but they agree in genesis and distribution, in 

 the differentiation of their cellular layers, and in their 

 totality, with the Gastrula phases of invertebrate ani- 

 mals. 



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 verte- 

 brate from invertebrate animals is supported by a second 

 and no less important discovery by the Russian natural- 

 ist. It is, that during their development a number 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 transformation. At the orifice 



