The Embryolögy of Patella. 
19 
in contact with the anterior cells of the mesenteron; as the latter 
contracts towards the posterior and dorsal portion of the embryo 
the oesophagus keeps pace with it and describes in its course a more 
or less perfect semicircle; PI. Y, Fig. 58. The thickened ventral 
wall of the oesophagus close to the mouth produces a single median 
invagination, which probably gives rise to the radula, although 
I have not found embryos far enough advanced to follow its 
further changes. 
The mesoblastic chords, described in the last section, 
dnring the present stage iucrease in length reaching forward as 
far as the velum, at which point the cells, as they increase innumber 
by division, no Jonger retain their former shape and intimate 
connection with each other but become isolated and at the same 
time assume elongated forms with pointed ends; PL IV, Fig. 52 
The posterior portions of the mesoblastic chords remain for a 
long time intact being composed of two rows of cells anteriorly, 
while posteriorly they consist of a single row of compressed cells. 
The primitive mesoblast cells are situated at the very posterior 
extremity of the chords and are still easily recognized as such by 
their superiority in size. 
As the development of the embryo proceeds, a portion of the 
mesoderm cells, which have become freed from the anterior ex¬ 
tremity of the mesoblastic chord, moves towards the dorsal surface 
of the mesenteron, there forming a layer of cells which gradually 
grows toward the ventral side. Half a dozen or more of these 
cells become greatly elongated and, converging toward a point 
on the dorsal surface, become attached to the shell and form the 
muscle cells which serve to draw the embryo into the shell, when 
the latter has attained sufficient proportions. Another group of 
mesoderm cells extends forward and ventrally, surrounding the 
oesophagus and forming a specially large Collection of cells above 
and at the sides of the oesophagus and around the invaginated 
pits on either side of it, which probably give rise to the auditory 
sacs. At the same time a few scattered cells arrange themselves 
around the inner wall of the velum; PI. V, Fig. 60. A few elon¬ 
gated cells usuallv connect the mesoderm cells encircling the 
oesophagus with those upon the velum. A special chord of such 
cells, distinguished by their greater size is always found on 
either side connecting the oesophagus with the velum; PI. V, 
Fig. 60. Besides these a few free, deeply stained and perfectly 
spherical cells are always to be found in the spaces between the 
12 * ( 167 ) 
