434 



TEMP. -MAJOR H. M. EVANS ON 



invagination becoming deeper as one traces it posteriorly. 

 Mici'oscopically, one can observe that this invagination is carried 

 •out by a specialised portion of epithelium. Where the spine 

 begins to separate, the flattened epidermal layer and mucous 

 • cells coveiiiJg the tail are replaced by a layer of columnar cells 



Texb-fioure 2. 



o^p.s. 



Tri/gon pastinaca. 



Portion of gland of groove, showing follicles full of secreting cells, 

 and central canal eniptj'. 



c.c. Central canal or duct (empty), c.t. connective tissue, d.g. ducts of gland. 

 f.g. follicles of gland, m.f- muscle-fibres, n.c.c. nipple of central canal. 

 o.p.s. osseous portion of spine. 



with superimposed layers of rounded cells, I'esting on a basement- 

 membrane in which is a layei' of large deeply pigmented cells. 

 This peculiar layer of epithelial tissue gradually grows inwards, 

 i. e., towards the middle line on either side, and inserts itself 

 ■between the tissues occupying the grooves and the tail. 



