102 WILLIAM BATESON. 
thickness is, so to speak, inversely proportional to that of the 
cellular tissue of the notochord, which becomes thinnest in the 
region where they attain their maximum size. This, therefore, 
suggests that they are formed at the expense of the notochord. 
An analogy moreover at once occurs of the secretion of such 
a substance from notochordal tissue in the case of Amphioxus, 
in which discs are deposited of very similar histological 
character to these masses in Balanoglossus. These two rods 
posteriorly bend downwards and then slightly forwards lying 
in the hypoblast. They are therefore each cut twice in sections 
through this part of the body. 
Behind the end of the notochord are the gill-slits, which are 
still only circular pores leading to the exterior (fig. 43). The 
lumen of the gut in the branchial region is (in contracted 
specimens) much suppressed, and its ventral side, however, 
always is grooved, and in this groove the cilia which line the 
branchial region are well developed. 
When the animal contracts, the branchial region of the gut 
posteriorly projects over the front of the digestive region on 
the dorsal side (fig. 44). 
The digestive region is quite distinct from this point 
in development onwards. Its cells have the appearance shown 
in fig. 45, being large cells with amoeboid processes containing 
large granules. From the back of the digestive region the 
intestinal region is now marked out. Its walls are thinner, 
and the cells composing it are long and ciliated at their inner 
ends. The anus is a large aperture, permanently open when 
the animal is expanded, situated dorsal to the tail. There is 
no epiblastic proctodzum. 
Mesoblastic Structures. 
Anterior Body Cavity. The mesoblast of this tract may 
now be divided into two parts—(1) a peripheral portion 
which lines the body walls of the proboscis, and (2) a central 
portion which is pushed in by the forward growth of the 
notochord ; between these two portions there is a body cavity 
which retains a clear central space throughout life. 
