1898 - 99 .] Mr T. H. Bryce on Duplicitas Anterior. 
627 
that the study of a series of sections of an early stage of the con- 
dition can provide a definite answer. 
There has been much discussion over the meaning and fate 
of the primitive streak. Many authorities, notably Balfour, have 
allowed it no part in the formation of the embryo, others have 
regarded it as gradually converted from before backwards into the 
primitive axis, but considerable divergence of opinion has prevailed 
as to the exact share it takes in its formation. Recently, how- 
ever, Kopsch (x.), by direct experiment, has demonstrated that 
the whole streak in birds is used up in the formation of the em- 
bryo, only the anterior part of the head being laid down in front 
of it. Thus variations of the primitive streak acquire increased 
significance. Multiple primitive streaks on a single blastoderm 
have been described by Allen Thomson (xi.), Bruckhardt (xii.), 
and Mitrophanow (xiii.) — but no case has yet been described, so 
far as I know, of an undoubted simple single streak bifurcated 
in front. This is the condition, I believe, in the present case, 
the only alternative explanation being that there are three radial 
streaks, which I can hardly accept as possible in view of the 
appearances presented by the series of sections through this 
region. 
If one attempted to interpret the condition in the light of 
Hertwig’s “ Urmund ” theory (xiv.) or Kopsch’s (x.) modification 
thereof, and regarded the primitive streak as homologous with the 
drawn-out gastrula mouth, the lips of which come together to form 
the axis of the embryo, such a Y-shaped streak would form an 
interesting parallel to the Y-shaped figures which 0. Schultze (xv.) 
describes as resulting from abnormal gastrulation in frog’s eggs 
treated by compression and rotation, and which resulted in tad- 
poles showing duplicitas anterior. 
But, theory apart, the primitive streak being the anlage of the 
embryonal axis, the final constitution of the double form will be 
determined, in its variety and degree, by the relations of the 
streaks (or the limbs of the streak) to one another, and by the 
measure in which the growth of parts is interfered with, owing to 
the approximation of the cell masses constituting their anlagen, as 
shown by Dareste. This is not “fusion” strictly, however, for 
the anlagen of intermediate organs are composite, fused, from their 
