390 PROCEEDINGS OF THE ACADEMY OF [April, 
outward into the velar area branching extensively. Fine interlacing 
fibers are also found in the foot in older stages. 
Returning to the period marked by fig. 105, the dorsal retractor 
muscle is seen to be a short thick strand of cells extending from the 
shell region to the enteron near the position of the liver. It is here 
already branched and runs along the sides of the alimentary canal. 
The right retractor of the foot is, as shown, a very heavy cell strand 
which unites the foot with the lower dorso-lateral portion of the shell. 
A view from the left side would show a muscle occupying a similar 
position, but in this case much thinner (fig. 107 shows their relative 
sizes at a slightly later stage). Even at this early period the dorsal 
retractor is posteriorly attached to the left of the median line. 
Bearing in mind the distinction of Lillie and others between primary 
mesoblast (ento-mesoblast) and secondary mesoblast (ecto-mesoblast 
or larval mesoblast), the attempt has been made to distinguish between 
these two sources of muscular tissue in the developing larva of Fiona, 
with, however, but partial success. The velar retractors, which lie in 
the region of the head vesicle, are formed from secondary mesoblast. 
Those cells which we have seen cut off from the third quartet in the 
two anterior quadrants lie in the antero-lateral region of the gastrula, 
and may for some time be distinguished from the primary mesoblast 
cells. When at an early period spindle-shaped muscle fibers appear 
in this region, their origin from these cells can scarcely be doubted. 
The component elements of the dorsal retractor are hard to distinguish. 
When this muscle first appears at a stage about midway between figs. 
104 and 105, several large cells lie wedged in between the rounded 
wall of the enteron and the ectodermal area in the upper region of the 
shell-gland. The evidence is strong that these cells at least are from 
the primary mesoblasts. At this time, however, other cells extend 
along the enteron, connecting the compact posterior group with the 
loosely lying spindle-shaped elements of the velar retractors. They 
doubtless help form the more anterior portion of the dorsal retractor 
and, lying as they do so close to where secondary mesoblast was formed, 
may be derivatives of it. The two retractors of the foot and the inter- 
lacing fibers of that organ itself are doubtless composed of cells which 
have come from 4d. From the above account it is seen that a true 
‘“‘larval mesoblast”’ is found in Fiona, since much at least of the mus- 
culature of the velum, a purely larval organ, is derived from this 
secondary mesoblast. 
No organ in any way comparable to a larval heart is to be found in 
the oldest veligers which I have studied. 
