STRUCrURE AND DEVEL(;rMENT OE COELUPLANA. 19 



the transverse axis of the body, with two relatively broad parts on each 

 side of the narrowest median part (PL 2, fig. 2). The median part 

 (" Mittelstreifen ", Hertwig, '80, Samassa '92) is spindle-shaped in the 

 dorsi-ventral view, it is made up of the tentacule-root and the part 

 of the basis-epithelium overlying it. The lateral parts (" Seitenfeld ", 

 Hertwig, '8o; " Seitenstreifen", Samassa, '92) are oblong in shape and 

 are formed by the part of the basis-epithelium which does not cover 

 the tentacle-basis. The length .of the three parts is nearly the same. 

 The proximal end of the lateral parts is placed just outside of the 

 transverse wall of the pharynx, while that of the median part attains 

 somewhat nearer the centre of the body and is wedged in the median 

 cleft of the pharyngeal wall to be described afterwards. The tentacle- 

 stem is inserted at the end of the proximal one-third of the length of 

 the median part of the tentacle-basis. 



The basis-epithelium (PI. 2, fig. 2; PI. 3, figs. 5, 6, 8; b. ep) is, 

 after all, that part of the epithelium of the tentacle-sheath which occupies 

 the entire floor of the ventral compartment of the sheath. It differs 

 from the epithelium of the other parts of the sheath in being considerably 

 thicker, consisting, as it does, of very small cells superposed in more 

 than forty cells deep. These cells all show a strong affinity for basic 

 dyes, and the structure has a very conspicuous appearance in stained 

 sections. The conspicuity is enhanced by the structure showing often 

 some foldings (PI. 3, fig. 6, b. ep). The differentiating process of the 

 tissue of the basis-epithelium advances largely centripetally: the growing 

 zone is situated in the marginal regions, and the tissue becomes more 

 differentiated towards the central region, where it passes almost insensibly 

 over into the epithelium of the tentacle-stem or into the colloblasts. In 

 the marginal regions the basis-epithelium is very thin and is formed by 

 extremely minute cells; it stains uniformly very intensely with haematoxy- 

 lin. The epithelium becomes thicker towards the central region, and 

 the individual cells grow larger and are better differentiated. The 

 proximal and distal halves of the basis-epithelium differ from each other 

 in their ultimate fates, the former developing into the epithelium of the 

 tentacle-stem and the latter into colloblasts. Often there exist some 

 isolated patches of young tissue among the older. 



The tentacle-root (PI. 2, figs. 2, 4; PI. 3, figs. 3-6, 8; /. r) is a lateral- 

 ly compressed fan-shaped structure; it consists of embryonal muscle-fibres 

 converging into the tentacle-stem. It is divided into two symmetrical 

 halves by a kind of septal band of a gelatinous tissue situated in the 



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UJ I L I B R A R Y 



