364 TUNICATA. 



It must be mentioned that at a later stage (especially during the transforma- 

 tion connected with fixation) the mesenchyme becomes apparently enriched 

 by elements which, when the entodermal strand (Fig. 169, s) and the larval 

 nervous system disintegrate, become free. Kowalevsky thought that these 

 cells became changed into blood-corpuscles, and later, Seeliger (No. 50) 

 utilised this fact in constructing his theory of the budding in compound 

 forms. It, however, appears doubtful to us whether these elements take any 

 part whatever in the formation of the organs of the adult or of their buds or 

 whether they do not rather, after reaching the blood, undergo degeneration. 



The alimentary canal. The rudiment of the alimentary canal is 

 derived from the archenteron by the separation of the mesoderm 

 bands and the chorda-strand. In early embryonic stages (Figs. 162, 

 163, p. 351) it consists of an anterior pre-chordal dilatation (en) and, 

 following this, of a narrowed part lying already below the chorda but 

 still belonging to the trunk-region. This narrowest part is directly 

 continued into the sub-chordal entoderm-strand of the caudal region 

 (en') which must be regarded as the intestinal rudiment of this 

 part of the body. 



Since the mesoderm and the chorda (as seen above, p. 350) arise 

 by a process of folding from the dorsal wall of the archenteron, the 

 dorsal wall of the intestine is defective at this point ; this gap be- 

 comes closed in later stages by certain cells as described above 

 (p. 350). This defect is only found in the trunk-region in the 

 posterior narrowed part of the alimentary canal, as the rudiment of 

 the caudal region undergoes no further advance in form and, on the 

 other hand, the pre-chordal section of the intestine takes no part in 

 the foldings which give rise to the mesoderm and the chorda. This 

 narrowed sub-chordal trunk-portion of the intestine, after the gap 

 just mentioned has closed, forms a blind diverticulum projecting 

 backward (Fig. 167 A, d, p. 356), the end of which, according to 

 Kowalevsky, at those stages in which the caudal section becomes 

 more sharply marked off from the trunk, bends slightly towards the 

 dorsal side, thus severing its connection with the cellular entoderm- 

 band of rlie caudal region (sc). In this way is introduced the de- 

 generation of this last part of the intestine mentioned above, which 

 leads to the cells of this band becoming disconnected and assuming 

 a resemblance to blood-corpuscles. We then find, beneath the chorda 

 in the caudal region, a cavity apparently filled with blood-corpuscles 

 and in direct communication with the spaces of the pseudocode. 



According to Kowalevsky, with whom the .majority of later 

 authors (Kupffer, Seeliger) agree, the branchial sac or pharynx 

 is derived from the pre-chordal dilatation of the intestinal rudiment, 



