104. WILLIAM BATESON. 
The position of this space is shown in fig. 45. It will be seen 
in the sequel that the mass of the secreting tissue of the gland 
is formed from the cells covering this space, and forming the 
sides of this central portion of the mesoblast. The space itself 
contains eventually but few of these secreting cells, and will 
be spoken of as the sac of the proboscis gland (fig. 52, gl. s.). 
The heart is as yet not represented. 
Middle Body Cavities.—The tissue lining these cavities 
does not exhibit more than a general progress of differentiation. 
Owing to the increased narrowing of the proboscis stalk the 
two anterior horns of the cavities are more distinct. In the 
dorsal and ventral mesenteries basement membranes occur. 
The cells lining these cavities are generally pyramidal or 
crescentic, some of them being radially directed and fusiform. 
Posterior Body Cavities.—The tissue of the posterior 
mesoblastic pouches has undergone the same proliferation and 
progressive differentiation as that of the middle cavities. These 
processes are not, however, quite so far advanced. The two 
horns (peri-hemal cavities) which began to grow forwards 
above the gut in the one-gill larva are now much more de- 
veloped. They now extend into the back of the collar region, 
in front of the first gill-slit. They are filled more or less with 
loose mesoblastic tissue containing a few fibres. As before 
stated, in the mesentery between them is formed the dorsal 
blood-vessel. This structure appears as a split in the 
mesentery, and is as yet quite empty in preserved animals. 
This split extends already through the whole course of the 
perihemal cavities. On the ventral side also a split is formed 
in the lower mesentery of the posterior body cavity to form 
the ventral blood-vessel. There is as yet no connection 
between the dorsal and ventral vessels. 
From this point the details of the subsequent development 
and anatomy of the parts will be given in the section dealing 
with the separate organs ; but, before doing so, it may be well 
to describe briefly the general course of the later history of the 
internal structures. 
Notochord.—As will be seen, this structure increases 
