478 
Fig. 5. Transverse section of portion of inner vesicle from the dorsal side in 
posterior region, indicating origin of rudiment of the sexual organs, s. 7.5 im. v., inner 
vesicles; at lower right hand corner of figure cells are passing out of the wall probably 
to become cells of the blood. (500 Diam.) 
Fig. 6. Portion of wall of inner vesicle, showing a cell, a, at an early stage of 
migrating into body-space. (500 Diam.) 
proceed from before backward, and as a result of this the atrium is 
found entirely separated from the primitive vesicle in the anterior 
region at a time when the folds stand widely apart more posteriorly. 
Eventually, however, the process is completed and instead of the 
original simple vesicle, we find the median or branchial sac surrounded 
by the U-shaped peribranchial cavity. 
By the time the peribranchial folds have begun to appear, the 
connection of the primitive vesicle with the inner tube of the stolon 
has become severed as a rule, although this is a very variable 
occurrence and may take place at an earlier or later period. 
The dorsal tube or hypophysis arises from a little string of 
cells which wander out from the extreme anterior end of the inner 
vesicle at quite an early stage. The evidence for this mode of origin 
is Just as clear as in the case of the pericardium; the boundary line 
is broken at many points on the wall; active cell-division is seen to 
be going on in the latter, and cells are frequently found actually 
migrating into the rudiment. It is very probable that here too cells 
of the blood take part in the formation, as they are seen adhering to 
the outside of the mass and apparently becoming incorporated with 
it. In fact no sharp distinction exists at early stages between the 
peripheral cells of the rudiment and cells lying free in the body space, 
as they pass gradually into each other and are identical in appearance. 
SS ir ee 
