„to the segmeutal involutions, aud others agaiu , thougb. this is 

 „not common , in a part of the mesoblast , which distinctly be- 

 „longs to the body-wall. (PI. XI. fig. 14 6). 



„The portiou of meseutery in which the primitive ova are 

 „most densely aggregated , corresponds to the future position of 

 „the genital ridge, but the other positions occupied by ova are 

 „quite outside this. Some ova are in fact situated on the outside 

 „of the segmental duet and segmental tubes, and must therefore 

 „effect a considerable migration before reaching their final posi- 

 „tions in the genital ridge on the inner side of the segmental 

 „duet. (PI. XI fig. 14 6.)" 



Wir seheu hieraus, dass Balfour alle grosseren Zeilen, welche 

 er beim Embryo im Epithel des Mesenteriums findet als echte 

 „Ureier" d. h. Geschlechtszellen betrachtet und dass er geneigt 

 ist, ihnen die Eigenschaft der aktiven Fortbewegung zuzuschrei- 

 ben. Hierüber sagt er weiter noch : 



(p. 134.) „In some of the lower animals e. g. Hydrozoa, there 

 „is no question that the ova are derived from the epiblast; we 

 „might therefore expect to find that they had the same origin 

 „in Vertebrates. Further than this, ova are frequeutly capable in 

 „a young state of executing amoeboid movements, and accord- 

 „ingly of migratiug from one layer to another. In the Elasmo- 

 „branchs the primitive ova exhibit in a hardened state an irre- 

 „gular form, which might appear to indicate that they possess 

 „a power of altering their shape, a view which is further sup- 

 „ported by some of them being at the present stage situated in 

 „a position very different from that which they eventually occupy, 

 „and which they eau only reach by migration. If it could be 

 „shewn that there were no intermediate stages between the pri- 

 „mitive ova and the adjoining cells (their migrating power being 

 „admitted) a strong presumption would be offered in favour of 

 „their having migrated from elsewhere to their present position. 

 „In view of this poasibility I have made some special investiga- 

 „tions, which have however led to no very satisfactory results." 



Den durch das Auffindeu von Uebergansformen zu liefernden , 



