16 MORPHOLOGY OP 



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 difi'erentiation 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, x). It consists in the prolongation of the ends of the 



* For reasons which will subsequently appear this term is somewhat in- 

 appropriate. 



