32 J. T. CUNNINGHAM. 
crescence is taking place, while z marks the posterior border of 
the primitive blastopore. 
The later History of the Periblast in Teleostei. 
At later stages of development, after the intestine is 
completed and the body cavity formed in front of the 
yolk, the periblast must be considered as belonging to the 
mesoblast, as, in fact, part of the splanchnopleure. Ryder 
(3) insists that the chief function of the yolk hypoblast or 
periblast is the formation of blood-corpuscles, which are 
budded off from it into the body cavity in Alosa, into the 
vitelline vessels in Salmo. I have not observed this myself 
yet, but I do not doubt that it takes place in the herring larva. 
In the newly-hatched herring there is a space which must b 
regarded as belonging to the coeloma, situated in front of the 
yolk, and in this cavity is situated the heart, the hinder end of 
which is in open communication with the cavity. There is 
nothing separating the cavity from the nucleated periblast still 
enclosing the remnant of the yolk, and this fact, together with 
the formation of the blood-cells from the periblast, compel us 
to consider the periblast at this stage as part of the splanchno- 
pleure. In many animals, e.g. Echinoderms, mesoblast cells 
are budded off from the hypoblast ; in all animals the mesoblast 
must be originally derived from epiblast and hypoblast, and the 
conversion of part of periblast in Teleostei into splanchnopleure, 
without change in its plasmodial constitution, is a phenomenon 
homologous with the budding of mesoblastic cells from hypo- 
blast in lower forms. It is probable that in other Vertebrates 
also the splanchnopleure is derived partially from the yolk- 
cells. Thus the nucleated yolk in Vertebrates contains, in addi- 
tion to elements destined to form the floor of the intestine, also 
elements destined to form splanchnopleure and blood. The 
explanation of the continuity between the body cavity and the 
cavity of the auricle in Clupea and other Teleosteans has not 
yet been found, though the fact has long been known. Whether 
the segmentation cavity is ever actually continued into the 
