22 MORPHOLOGY OP 



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 uotochordal 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 extended, situated dorsal to the tail. There is 

 no epiblastic proctodseum. 



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. 



