308 ARTHUR DEXDT. 



intestine, just before reaching the anus, are quite indistinguishable 

 from this roof. 



I have thus no decisive evidence to bring forward as to the manner 

 in "which the alimentary canal is formed ; but there appears to me to 

 be a strong probability in favour of the view that it is formed by in- 

 vagination, the inner, glandular layer of its walls being formed from 

 invaginated epidermis and the outer connective tissue layer from the 

 underlying dermis. 



Five days. — This specimen agrees closely with the last. The ali- 

 mentary canal appears to be in nearly the same condition. It is very 

 small and almost solid, the inner layer of its walls being composed of 

 minute, deeply staining, nucleated cells, more or less rounded in shape 

 and showing a slight tendency to become columnar in places, and 

 being histologically indistinguishable from the tissue covering the 

 general surface of the disc. Here again the alimentary canal appears 

 to have been formed by invagination, accompanied by rapid prolifera- 

 tion of the cells of the epidermis. In the specimen described this 

 proliferation has formed a thick mass of minute cells projecting on the 

 surface at one side of the mouth. No anal cone is as yet visible. 

 The terminal portion of the intestine touches the roof of the visceral 

 basin, but I found no definite opening through; but it must be remem- 

 bered that the anus, on its first appearance, is so minute a structure 

 that it might easily escape observation, especially in imperfectly pre- 

 served sections. 



No ambulacral nerves or canals are as yet visible round the mouth. 

 A number of new ambulacral pores have been formed, apparently by 

 invagination. The epithelium lining them is at first not distinctly 

 columnar. 



Nine days. — Externally the boundaries of the ambulacral grooves 

 are seen to have met and formed a pentagon around the mouth. They 

 enclose a slightly depressed area with the mouth in the centre, and 

 appear as thickish white ridges, in some places notched, showing where 

 lappets are beginning to regenerate. 



It appears to me that the ambulacral grooves are, from the first 

 commencement of regeneration, left as areas along which the thicken- 

 ing of the regenerating tissue is not so great as elsewhere. This 

 thickening takes place centripetally, in five distinct areas, one in each 

 interradius ; as these grow inwards the ambulacral grooves and the 

 central depression around the mouth are left as less thickened portions. 



