290 
SIDNEY F. HARMEE. 
the vestibular invagination ( v .), described later. The band 
consists of a series of branched cells, lying in the blastocoel 
(“ primary body cavity 55 of Hatschek), in which are seen, in 
addition, three mesoblast cells, whose origin is uncertain. In 
the next stages the mesoblast consists of cells with anastomosing 
processes (figs. 41 and 42), scattered irregularly through the 
blastocoel, but I have never observed anything corresponding to 
a splitting of the mesoblast into splanchnic and somatic layers. 
In still later stages the mesoblast occurs mainly at the sides 
of the body, the more median regions being occupied by the 
alimentary canal, “ dorsal organ, 55 and “ vestibular invagina- 
tions. 55 The mesoblast cells, however, fill up almost all the 
interspaces between these structures, so that the mature lava 
is practically a solid mass of closely-packed tissues, this fact, 
of course, rendering it a difficult undertaking to distinguish 
the nature of all its constituent cells. 
The disposition of the mesoblastic bands (which at first are 
situated near the ventral surface), and their relation to the 
pole-cells (which originate at the sides of the blastopore, and 
are subsequently found at the sides of the anus), seem to me 
to support the view that the blastopore and anus are identical 
in position. 
The alimentary canal of the embryo is completed at an early 
stage, and subsequently undergoes no very striking changes 
until the larva becomes free. It consists in advanced embryos 
of the following regions, all distinctly marked off from one 
another : (i) the oesophagus (fig. 52, ce.) doubtless formed as 
a stomodseum; (ii) the stomach and intestine (st. and int .) 
passing quite gradually into one another. Dorsally and late- 
rally the stomach is lined by low hypoblast cells, ventrally by 
cells which are high and columnar, nucleated peripherally, and 
containing yellow spherules ; these cells, in fact, are the precise 
equivalent of the “ liver-cells 55 of the adult; (iii) the rectum 
( rec.) } separated from the intestine by a constriction, and pro- 
bably formed as a proctodaeum. The anus opens on a well- 
marked rectal cone projecting into the vestibule. 
The alimentary tract of the larva thus resembles in every 
