The Prophase in the Ovigenesis and the 
Spermatogenesis of Planaria lactea 0. F. M. 
(Dendrocoelum lacteum Oerst.) 
ßy 
George Arnold. 
University of Liverpool. 
With 1 figure in the text and plates XXII — XXIII. 
The following observations deal with the maturation of the eggs 
of Planaria lactea, from the resting stage, up to the late prophase, 
(Stade prespirematique a novaux reticules to Stade postspirematique 
a noyaux diplotenes of Gregoire,) and with the whole of the Sper- 
matogencsis. It is the object of this paper to show that in PI. lactea , 
the formation of the heterotypic gemini is not brought about by the 
longitudinal approximation of somatic univalent chromosomes in tlie 
early prophases, but that the spireme Segments appear from the very 
commencement, that is at the stage at which they can tirst be defi- 
nitely distinguislied, in a number which is equal to half that of the 
somatic chromosomes, and that the longitudinal split which makes its 
appearance in these Segments, is not to be interpreted as the space 
between two univalent segments which have United along their lengtli, 
but is merelv a temporary condition, and foreshadowing the plane of 
division of the daughter chromosomes of the 2nd. division; i. e., the 
homotype chromosomes. 
This view has been strenuously opposed by H. von Wjxiwarter, 
Gregoire, Janksens, Molle and others. Looked at by itself, it is 
Archiv f. Zellforschung. III. 29 
