18 MOEPHOLOGT OF 



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. 45). 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 a 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. 



