24 MORPHOLOGY OF 



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 diflerentiation as that of the middle cavities. These 

 processes are not, however, quite so far advanced. The two 

 horns (peri-haemal 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 

 periheemal 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 



