204 ZOOLOGY. 



from the upper part of the germ. This is the central ner- 

 vous system, and in the cavity are subsequently developed 

 the sense organs. We thus see, says Kowalevsky, a com- 

 plete analogy m the mode of origin of the nervous system of 

 the Ascidians to that of the vertebrates, the nervous cavity, 

 where the embryo is seen in section, being situated above 

 the digestive cavity in both types of animals. 



The next important stage is the formation of the tail. 

 The pear-shaped germ elongates and contracts posteriorly 

 until of the form indicated at Fig. 138, B. At this period 

 appears the axial string of nucleated cells, called the chorda 

 dorsalis, as it is homologous with that organ in Aniphioxus 

 and the embryo of higher vertebrates. The nervous system 

 consists of a mass of cells extending halfway into the tail 

 and directly overlying the chorda, but extending far beyond 

 the end of the latter as seen in the figure. The nerve-cav- 

 ity {£, n) after closing up forms the nerve-vesicle, a large 

 cavity (Fig. 139, a), in which the supposed auditory organ 

 (e) and the supposed eye \a) arise ; tliis cavity finally closes, 

 and the sense-organs are indicated by certain small masses 

 of pigment cells in the fully grown Ascidian larva. 



As the embryo matures, the first change observed in the 

 cord is the apjjearance of small, refractive bodies between 

 the cells. Between the neighboring cells soon appear in the 

 middle minute highly refractive corpuscles Avhich increase 

 in size, and press tlie cell-contents out of the middle of the 

 cord. After each reproductive corpuscle grows so that the 

 central substance of the cell is forced out, it unites with 

 the others, and then arises in the middle of the simple cel- 

 lular cord a string of bodies of a firm gelatinous substance 

 which forms the support of the tail. After this coalescence 

 the substance develops farther and presses out the proto- 

 plasm of the cells entirely to the periphery. The cord when 

 complete consists of a firm gelatinous substance surrounded 

 by a cellular sheath which is formed of the remains of the 

 cells originally comprising the rudimentary cord. The cells 

 lying under the epithelial layer form a muscular sheath of 

 which the cord (Fig. 138, c) is the support or skeleton. 



The alimentary cavity arises from the primitive cavity 



