204 Edwin G. Conklin. 



b. Notochord. 



Chabry supposed that the notochord arose from both the 

 anterior and posterior quadrants of the egg. Castle ('96) held that 

 a single pair of cells of the posterior quadrants, B*", "the posterior 

 chorda fundament," were the only cells of the posterior quadrants 

 which entered mto the formation of the notochord. I am of the 

 opmion that this cell is a mesenchyme and not a chorda cell (see 

 Conklin, '05^), but even if it should be found to be a chorda cell 

 It IS only one cell of nine on each side of the mid line which give 

 rise to that structure, while eight pairs of chorda cells come from the 

 anterior quadrants. Certain it is that no trace of a notochord ever 

 arises from the posterior cells when they are isolated, whereas 

 chorda cells always arise from isolated anterior cells, though a 

 notochord is rarely formed in such cases. Chabry describes 

 (p. 294, Fig. 118) an anterior two-quarters embryo in which a 

 naked chorda was seen in the perivitelline space outside the body 

 of the embryo; such a case somewhat resembles the one shown in 

 my Fig. 72. However, in every other instance which I have 

 observed the chorda cells of an anterior embryo do not give rise 

 to a notochord, but after escaping from the body of the embryo 

 lie free in the perivitelline space as scattered cells. (Figs. 52, 

 66-70.) 



But while a notochord is rarely or perhaps never formed in an 

 anterior embryo and never in a posterior one, it is invariably 

 found in a right or left one, and the figures of Chabry and Driesch 

 as well as my own show that the process of formation is essentially 

 the same as in a normal embryo. Chabry indeed believed that 

 the notochord was primitively double and that half of it arose 

 from each lateral half of the egg. He speaks of the fact that in 

 Ascidia and Botryllus it is composed of a double row of cells and 

 Crampton also refers to the fact that in the normal ascidian tad- 

 pole there are two rows of chorda cells, whereas Driesch has well 

 said that in its fully formed condition the ascidian notochord is 

 normally composed of a single row of cells. I find, as did Driesch, 

 that the notochord of a lateral embryo is formed bv interdigitation, 

 just as in the normal embryo, but I also find, as opposed to Driesch 

 that the notochord is never formed from any cells save the chorda 

 cells which come from the posterior part of the gray crescent. 

 Furthermore, my observations show, as did Chabry's, that the 



