Sonic Phciiouu'iia of Rcr^eiii'r.itimi in Lininudriliis aiid rel.ilcrl Forms. 431 



round, tlu- chruiuatin grauules are eveuly distributcd tliroughout aiid 

 tlie nuclcülus is merely a sniall black dot. The first change noticeable 

 is an enlargement of tlie nucleolus together with a widening of the 

 surrouiiding clear aroa, the chromatin being confined to the more 

 periplieral ])arts of tlie nucleus. At the stage under consideration 

 the nueleohis lias already attained an appreciabk' size. As yet there 

 is no Uligration of the enlarged cells but as the changes proceed, divi- 

 siou also occurs uutil finally the cells become so numerous that they 

 cau uo longer remaiu iu the ectodermal layer and consequently since 

 the body is uot elougating they are forced into the coeloni. 



In a posterior eud whicli regenerates uornially the body wall is 

 coutinually leugtheuiug and the ectoderin cells are being used iu the 

 forniation of regenerated stnictures such as the circular muscles and 

 the ventral nerve cord so that the enlarged ectoderm cells do not 

 accunuilate in quantities sufficient to force the cells into the coelom. 

 This nietaniorphosis necd not be confined to the scar region alone as 

 is shown in the case of the lateral anal opening (Fig. 21 and 22). 

 Here the enlargement occurs botli anterior and posterior to the point 

 at which the intestine touches the entoderm but migration has begun 

 only on the posterior side. The change in the ectoderm does not extend 

 over as great an area at the level at which the intestine touches the body 

 wall but below it, as shown in Figure 22, the change is more prominent. 



An advanced condition frequently presents a most remarkable 

 appearance. In one individual in which the ectoderm had thickened 

 so as to protrude beyond the body in the form of a bud all stages in 

 the metamorphosis can be observed and the whole bears a striking 

 resemblance to an ovary (Fig. 19). Cell boundaries are rather indis- 

 tinct but the nuclei are sharply outlined. Those nuclei nearer the 

 periphery are little larger than normal and proximal to tliem are others 

 showing various stages of enlargement, those nearer the coelom bear- 

 ing a striking resemblance to neoblasts, several of which are in the 

 immediate vicinity. The nuclei are approximately spherical and of 

 enormous size, some of them being almost as large as a normal ectoderm 

 cell, the nucleolus alone approaching the size of an ordinary nucleus. 

 The clear area surrounding the nucleolus has increased so greatly as 

 to occupy most of the nucleus, the latter in turn filling the greater 

 part of the cell. In some cases the area over which this change takes 

 place may be considerably larger and the ectoderm about the entire 

 ventral portion of the wound may be actively dividing and migrating 

 into the body cavity. 



Zeitscliritt l. wissensch. Zoologie. XeV. Bd. 28 



