96 WILLIAM BATESON. 
tinued backwards into two lateral horns. The mass of cells 
which now arise at the original point of separation, therefore, 
constitutes a continuation of the division between these two 
horns into the anterior simple cavity (fig. 14). As will 
shortly be seen, this division of the back of the anterior cavity 
into two is correlated to the forward growth of the notochord. 
In this septum, which is composed of roundish, hexagonal 
cells containing many granules (fig. 16), appears the first 
rudiment of a peculiar organ which subsequently is a con- 
spicuous and characteristic structure in the proboscis of all 
the Enteropneusta, viz. the proboscis-gland (“ heart” of 
Spengel'). This gland has at first the relations shown in 
fig. 10, gl. It consists (Stage F) of a triangular mass of loose 
tissue containing nuclei in which but few cell-outlines can be 
seen, and would appear to be formed by a sort of degeneration 
of the mesoblast of the septum. As yet it contains no cavity. 
Immediately behind and ventral to it is the anterior end of 
the notochord (fig. 17). 
Fig. 18 is taken through the posterior apices of the meso- 
blastic horns behind the gland. 
Middle Body Cavities.—These are (Stage E) a pair of 
simple cavities divided by a dorsal and a ventral mesentery, 
completely closed from both the anterior and posterior cavities, 
as they remain throughout life. They are lined by round or 
crescentic mesoblast cells. As the proboscis-stalk is con- 
stricted, the anterior parts of these cavities are compressed. 
into two forwardly directed horns. The horn of the left side is 
shown in fig. 18. From an early period it projects in front of 
that of the right side. 
In Stage F a histological differentiation occurs in the 
splanchnopleure at the place of union between that part of the 
hypoblast which is destined to form the notochord and the 
lower section of the pharynx: This appearance is shown in 
fig. 25, #). It consists in the prolongation of the ends of the 
1 For reasons which will subsequently appear this term is somewhat in- 
&pprop riate. 
