nervous-system is derived from the ectoderm, and first ap- 

 pears as a cord of cells lying close against the ectodermal 

 wall. Their description, however, is very incomplete and 

 unsatisfactory. 



Our more recent knov^rledge of the subject is due to 

 the researches of Pizon (22), Oka (20), and Hjort (8) on Bo- 

 tryllus , of Salensky (27) on Distaplia, and of Gaullery (1) 

 on Glosophorim and Diplosona. The first tnree authors, al- 

 though they are in agreement concerning the origin of the 

 dorsal tube, differ widely in respect to the derivation of 

 the ganglion. According to all tnree, the dorsal tube in 

 the Botryllus bud arises as an anteriorly directed evagina- 

 tion of the dorsal wall of the branchial sac, ending blindly 

 in front, but freely opening into the branchial cavity at its 

 posterior extremity. This tube grov/s forward, and its ante- 

 rior end fuses with the wall of the branchial sac, whose 

 cavity is then put into communication with the lamen of the 

 tube, while the posterior connection becomes obliterated. 

 The definitive opening of the hypophyseal tube is therefore 

 secondary. So far v/e find these authors agree, but it is 

 quite otherwise when we come to consider the origin of tne 

 ganglion. 



Pizon maintains that the ganglion of the bud is de- 

 rived directly from a fine nerve-string, which grov/s out from 

 the ganglion of the parent-bud, or in the first from tnat of 

 the larva, into the young bud. His view is not based on ac- 



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