The line of separation between the rudiment and the 

 wall of tne vesicle is seen to be perfectly distinct and 

 clearly marked, and there is no evidence whatever of prolif- 

 eration of endodermal cells at any point; Fig. 29, a. Du- 

 ring the early stages of development the cells which are to 

 form the dorsal tube, and many of the mesodermal cells, are 

 absolutely identical in appearance, and exui.At the same 

 amoeboid character. So gradual is the transition from the 

 free mesodermal cells to the cells of the rudiment, that it 

 is at first impossible to say where the former end, and the 

 latter begin, Pig. 29, b. 



I believe, therefore, that there is no evidence 

 whatevejr for holding that the dorsal t_ube does not aris_e frojn 

 free mesodermal cells of t^he blood. 



By further additions from outside and by active 

 cell-multiplication -.vithin the mass, the rudiment gradually 

 increases in size; its cells become more closely packed to- 

 gether, and soon form an elongated solid cord lying close 

 against the dorsal wall of the vesicle in the anterior end 

 of the bud. Fig. 29, c^. Fig. 29, a_, _b and _c illustrate the 

 development up to this point; a is drawn from the same sec- 

 tion as Fig. 28, h and _g from the series to which Figs. 10 

 and 11 respectively belong. It has been stated above that 

 the anterior portion of the inner vesicle is only slightly 

 involved in the displacement already described, but that it 



is to a certain extent, is proved by the fact that the rudi- 



first appears 

 ment of the dorsal tube not exactly in the mid-dorsal line, 



A 



■49- 



