306 ARTHUR DENDY. 



muscles on the floor of the visceral basin are both completely hidden 

 by the newly formed tissues. The line of tear is marked by the 

 sudden stopping of the lappets and pigment spots along the borders 

 of the ambulacral grooves. 



In sections we see that the surface of the regenerating cushion of 

 tissue is covered by a thin, deeply staining layer, which is apparently 

 formed by the arching over, and ingrowth of the edges of the injured 

 area. This layer is formed of two parts, an upper layer, which is 

 probably epidermal, and a lower dermal layer. The deeper, connec- 

 tive tissue layer is continuous with strands of the same substance 

 which have grown out from the floor of the visceral basin. In the 

 centre, above the chambered organ, the superficial layer of this regene- 

 rating visceral mass is still incomplete; so that on looking at it from 

 above two or three little holes are visible in the centre, leading down 

 to the chambered organ. The regenerated cushion of tissue is still 

 very thin, averaging in thickness only about half a millimetre. 



Eegeneration, then, appears to commence in two chief ways; (1) 

 by a series of outgrowths from the thin layer of connective tissue 

 which forms the floor of the visceral basin ; (2) by an ingrowth of 

 connective tissue and epidermis from the edges of the injured area, 

 forming a roof to the visceral basin. 



Three days. — Of this date I have two specimens, of both of which 

 I have sections, but the one is so much further advanced in regene- 

 ration than the other that I shall treat them as two separate stages, 

 taking the less advanced one first. 



(a) The new visceral mass is seen to have grown and has thickened 

 considerably. The openings leading down to the chambered organ 

 are now closed over. In the centre of the regenerating visceral 

 cushion is a small opaque papilla, probably due to thickening of the 

 epidermis preparatory to the formation of a mouth, but unfortunately 

 the sections are not sufficiently well preserved to decide this point. 

 Sections show that regeneration has proceeded as far as the formation 

 of a cushion of loose connective tissue, much thickerthan at two days 

 and covered by the deeply staining epidermic layer before mentioned. 

 The growth of connective tissue is especially strong underneath the 

 point where the mouth is subsequently to be formed ; so that we have 

 here a pillar of denser connective tissue, reaching from the top of the 

 rosette plate to the roof of the visceral basin and thickest just beneath 

 the epidermic layer. The central plexus is very well shown, but in 



