62 SUMMARY OF CURRENT RESEARCHES RELATING TO 



The endoderm-cells are partly invaginated in gastrulation, but the 

 gastrula is fiuisked by epibole. Indeed, the ectoderm wholly surrounds 

 the embryo, and no blastopore can be detected. Nor is there any archen- 

 teron ; the gastrula is solid, as Gardiner noted also in Polychoerus 

 caudatus. Then follows a remarkable process wherein the endoderm- 

 cells, after dividing repeatedly, undergo degeneration. Their cytoplasm 

 becomes finely granular and gathers round the nucleus, which is some- 

 what similarly affected. Thus results part of what is unfortunately called 

 the parenchyma of the Acoela. 



The ciliated ectoderm begins to move the embryo ; the nervous 

 system and sense-organs appear ; no trace of solid food is to be seen 

 in the parenchyma ; the endodermic material is at first the only source 

 of nutritive supply ; the zoochlorellaB suddenly appear in the peripheral 

 parenchyma ; they probably penetrate from the outside in time to save 

 the animals from dying of inanition. 



The term parenchyma covers two sets of elements : — (a) the peri- 

 pheral parenchyma, which is formed solely from mesoderm cells ; and 

 {&) the central parenchyma of degenerate endoderm-cells corresponding 

 to the gut of other Turbellarians. 



Regeneration in Turbellaria.* — Prof. W. Voigt seeks to distinguish 

 between normal regeneration, which is acknowledged to be frequent in 

 Turbellarians, and the occurrence of heteromorphosis (where the lost 

 part is replaced by one which is different in form and function). 



If the anterior part of the body of a Turbellarian be split in the 

 median plane and hindered from coalescing, each of the anterior halves 

 regenerates its missing side, and a Y-shaped form results, double ante- 

 riorly, single posteriorly. 



But if, after the median cut, a transverse one be made, say to the 

 right, so that the right anterior half is connected with the posterior 

 portion of the body only by a narrow isthmus, and if the healing of 

 the wound be hindered, a new tail is formed at the posterior end of the 

 anterior right half, necessarily almost at right angles to the longi- 

 tudinal axis. 



If the worm be cut obliquely almost across, so that the edges of the 

 wound may be made to gape more than is possible if the cut be straight, 

 then a new head may be regenerated from the posterior half, and a 

 new tail from the anterior half. 



If the oblique cut do not reach the middle line, the margin of the 

 wound towards the median plane heals, and the protruding knob 

 regenerates a head or a tail according to the direction of the cut, pro- 

 vided that the projecting portion be not too small. When it is very 

 small, there is either no regeneration of head or tail, or else the 

 incipient regeneration is interrupted by subsequent coalescence of the 

 cut edges. 



The author goes on to show that various circumstances may bring 

 about that the regenerated head is directed backwards ; but he doubts 

 whether any case of genuine heteromorphosis, with poles reversed, has 

 yet been made out. He does not deny the possibility, but he shows 

 how readily an apparent heteromorphosis may result, especially in 

 organisms which contort themselves so much as Turbellarians do. 

 * SB. Niederrhein. Ges. Bonn, 1809, pp. 25-31 (3 figs.). 



