400 LE RE VICE. [VoL. XIV. 
and there is no evidence whatever of proliferation of endo- 
dermal cells at any point (Pl. XXXII, Fig. 26, a). During the 
early stages of development the cells which are to form the 
dorsal tube and many of the blood cells are absolutely identical _ 
in appearance and exhibit the same amoeboid character. So 
eradual is the transition from the free blood cells to the cells 
of the rudiment that it is at first impossible to say where the 
former end and the latter begin (Fig. 26, 0). 
I believe, therefore, that all the evidence shows that ‘he 
dorsal tube ts derived from free amoebotd cells of the blood. 
By further additions from outside, and by active cell multi- 
plication within the mass, the rudiment gradually increases in 
size; its cells become more closely packed together, and soon 
form an elongated solid cord, lying close against the dorsal wall 
of the vesicle in the anterior end of the bud (Fig. 26, c). Fig. 
26, a, 6, and ¢, illustrates the development up to this point; a@ is 
drawn from the same section’ as) Pl, XXX) Fig. 133/07 and ec 
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 rudiment of the dorsal tube first appears not 
exactly in the mid-dorsal line, but a little to the left of this 
(Fig. 13, @.¢.7.). By the time the shifting of the vesicle has 
proceeded somewhat further than is shown in Fig. 11, the 
string of cells, which is now solid, has been carried up to the 
median plane. 
Very shortly after it has reached its definitive position, a 
lumen appears in the center of the rudiment throughout its 
entire length, and around this the cells become arranged into 
a one-layered epithelium (Pl. XXXII, Fig. 26, d and e). 
By following the course of development up to this point, it 
is seen how an epithelial tube does actually arise from free 
mesenchymatous cells, —a thing which Hjort has character- 
ized as most improbable. In criticising Seeliger’s view of the 
origin of the neuro-hypophyseal system in Clavelina, this author 
says (8: p. 602): ‘Die Wahrscheinlichkeit dafiir, dass ein 
Ganglion und ein epitheliales Rohr sich aus zusammengehauften 
