Ascidians . 



Fig. 50, represents a median sagittal section of a 

 bud before the complete separation of the peribranchial cav- 

 ity, and therefore before the dorsal tube has acquired an 

 opening into the branchial sac. The section passes through 

 the entire length of the tube, d . t_, v/hich is seen to be clo- 

 sed at both ends and made up of an epithelium of one layer. 



In my preliminary work on the budding of P.erophora, 

 already referred to, I made the statement that, "the ganglion 

 is formed by a thickening of the d_orsal wall of the tube, 

 which eventually becomes constricted off in the manner des- 

 cribed by Kjort for Botryllus, although in the latter it is 

 the _ventral v/all of the tube which gives rise to the gan- 

 glion." More careful study of very young stages, however, 

 has convinced me that the above is not an accurate descrip- 

 tion of the formation of the ganglion. 



After the communication between the dorsal tube and 

 branchial sac has been established, a few cells, identical 

 in appearance with mesodermal cells, are found adhering to 

 the dorsal surface of the tube throughout the greater part of 

 its length; this elongated, loose patch of cells constitutes 

 the rudiment of the prur-licn, £l. Fig. 31, a_. 



It is a difficult question to decide whether these 

 cells are entirely mesodermal, as their appearance indicates, 

 or whether they are derived from proliferation oi' tne wall of 

 the tube, fot- in many places the outline of the latter is 

 broken, and there is no sharp demarcation between the cells 



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