98 WILLIAM BATKHSON. 
alteration since the last stage. The epiblastic cells are some- 
what smaller and more closely packed. In the posterior 
regions, however, the ectoderm is thinner than in the 
younger animals, owing to the rapid growth which occurs at 
this time in the trunk region (fig. 46). Unicellular mucous 
glands occur at rare intervals in it. But in the lower layer of 
the skin at the posterior surface of the proboscis is formed the 
beginning of that network of nerve-fibres which is such a pro- 
minent feature in these regions of the body in later life. 
Though a network of this kind eventually is formed on the 
inner surface of the skin all over the body to a greater or less 
extent, it is as yet only to be seen in the base of the pro- 
boscis. The exact process by which this layer is deposited is 
not certain, but it would appear that cells of the inner layer 
elongate and form multipolar cells with long, thread-like, anas- 
tomosing tails (fig. 32). 
At this stage nuclei are still visible in this fibrous layer, 
though in later stages they have almost entirely disappeared 
from it (fig. 54, &c.). From this point in development on- 
wards these fibres constitute a perfectly defined layer of tissue. 
Projecting into it may be seen some of the fibres which were 
described as being formed from the mesoblastic lining of the 
proboscis cavity. These fibres are presumably supporting 
structures. Occasionally an appearance is presented as of an 
anastomosis occurring between them and the tails of the nerve- 
fibres. Whether this is really the case or not, it can scarcely 
be doubted that the muscle-fibres, which are now forming in 
the proboscis cavity, receive their innervation from the fibrous 
layer of the skin, to which many of them are attached, and 
thus such an anastomosis is not @ priori improbable. On the 
other hand, the structure of the mesoblastic fibres is rather in- 
dicative of a supporting than of a contractile function. But, 
as will afterwards appear, there are eventually present in the 
mesoblastic elements of these animals cells which present 
almost every shade of variety between undoubted contractile 
fibres and obvious connective tissue, so that it is by no means 
easy to determine the nature of these fibres with precision. 
