440 F. H. Krecker, 



derived from cells wliich havc migrated from the old mesoderm. Since 

 my results witli regard to the origin of the mesoderm at the posterior 

 end agree, in the main, with those of Iwanow I thought it important 

 to study its origin at the anterior end. For this purpose both Tubi- 

 j'ex and Limnodrilus were used and also the specimens of Lwmhncus 

 herculeus already mentioned. 



Regarding its origin Hepke says of the mesoderm » seinen Ursprung 

 allerdings an der Stelle nimmt, wo Ectoderm und Entoderm ineinander 

 übergehen; da aber die Bildungsstätte desMesoderms doch noch in einer 

 Zellenregion liegt, deren Elemente ihrem Aussehen nach ausgesprochen 

 ectodermalen Charakter tragen, so sehe ich mich veranlaßt, das Meso- 

 derm als einen Abkömmling des Ectoderms aufzufassen«. From the 

 latter portion of this statement it would seen that Hepke rests his con- 

 clusions mainly upon a similarity in appearance between ectoderm and 

 mesoderm cells. v. Wagnee, who likewise maintains that the mesoderm 

 is formed from the ectoderm, described what he terms »Reparations- 

 zellen«. These come from the ectoderm and their »Protplasmaleib ist 

 stets mehr oder weniger granuliert, der Kern groß, rund oder gestreckt, 

 hell und fast immer mit einem ansehnlichen Kernkörperchen ausge- 

 stattet, das sich intensiv färbt«. In addition to being formed at certain 

 Centers of proliferation he claims they also enter the coelom singly at 

 f)ther points. Various organs are derived from these cells and amojig 

 them the mesodermal structures. It is true that there is a proliferation 

 of ectoderm cells which migrate out into the coelom but they do not 

 liave a great variety of forms and, so far as I was able to observe, 

 they form only the ventral nerve cord, oesophageal commissure and 

 cerebral ganglia and do not wander about in the coelom (Fig. 25). 

 The proliferation of the cells is at first abundant on both sides of the 

 midventral line and then gradually progresses around the body to- 

 war4 the dorsal side where the two lateral bands of proliferation meet 

 at a point corresponding to the future position of the cerebral ganglia. 

 This practically agrees with the description given by Hepke (91), 

 V. Wagner (02) and Iwanow (0.3). About these neural primordia 

 and also for some distance posterior to them the coelom is filled witii 

 immerous cells of a great variety of shapes, some are round, the edges 

 of others have a tattered appearance but most of them are or less 

 spindle shaped. The nucleus is large and round with a distinct nucle- 

 olus. The cytoplasm is granulär, but does not stain readily with hema- 

 toxylin and eosin. These cells are not of ectodermal origin but come 

 from the disintegration of the old mesodermal eleraents present in 



