172 
F. M. BALFOUR. 
It is clear that Balbiani and Claparede have both failed to 
appreciate the importance of the organ, which my observa- 
tions show to be the part of the ventral thickening of the 
blastoderm where two rows of cells are first established, and 
therefore the point where the first traces of the future meso- 
blast becomes visible. 
Though Claparede and Balbiani differ somewhat as to the 
position of the organ, they both make it last longer than I 
do : I feel certainly inclined to doubt whether Claparede is 
right in considering a body he figures after six segments are 
present, to be the same as the dorsal organ of the embryo 
before the formation of any segments, especially as all the 
stages between the two appear to have escaped him. In 
Agelena there is undoubtedly no organ in the position he 
gives when six segments are found. 
Balbiani^s observations accord fairly with my own up to 
the stage represented in fig. 2. Beyond this stage my own 
observations are not satisfactory, but I must state that I feel 
doubtful whether Balbiani is correct in his description of the 
gradual separation of the procephalic lobe and the cumulus, 
and the passage of the latter to the dorsal surface, and 
think it possible that he may have made a mistake as to 
which side of the procephalic lobe, in relation to the parts 
of the embryo, the cumulus is placed. 
Although there appear to he grounds for doubting whether 
either Balbiani and Claparede are correct in the position 
they assign to the cumulus, my observations scarcely warrant 
me in being very definite in my statements on this head, but, 
as already mentioned, I am inclined to place the organ near 
the posterior end (and therefore, as will be afterwards shown, 
in a somewhat dorsal situation) of the ventral embryonic 
thickening. 
In my earliest stage of the third period there is present, 
as has already been stated, a procephalic lobe, and an indis- 
tinct and not very prominent caudal portion, and about 
three segments between the two. The definition of the 
parts of the blastoderm at this stage is still very imperfect, 
but from subsequent stages it appears to me probable that the 
first of the three segments is that of the first pair of ambu- 
latory limbs, and that the segments of the chelicerae and pedi- 
palpi are formed later than those of the first three ambula- 
tory appendages. 
Balbiani believes that the segment of chelicerse is formed 
later than that of the six succeeding segments. He further 
concludes, from the fact that this segment is cut off from the 
procephalic portion in front, that it is really part of the 
