Se eS eee ee ee el 
1886, ] Embryology. 825 
OBSERVATIONS ON THE EMBRYOLOGY OF SPIDERS.—Some work 
done during the past winter on the embryology of several species 
of spiders at the biological laboratory of the Johns Hopkins Uni- 
versity brought to light some facts of general interest. 
he origin of the lung book of the spider is particularly inter- 
esting, in view of the comparisons instituted between Limulus an 
the arachnids. From good longitudinal sections of the spider 
embryo before the disappearance of the abdominal feet, it appears 
that the lung book may fairly be regarded as an involuted ap- 
pendage or appendages. Before the involution of the abdominal 
appendages, the epithelium covering them assumes the characters 
of the epithelium of the lung book. At the same time the ap- 
pendages become less conspicuous, and slight folds appear on 
their anterior faces, 
By the complete involution of the abdominal appendages, and 
the increase in the number and distinctness of the folds on their 
anterior faces, a lung book would be formed with its laminz 
directed backwards. All the stages of the process of involution 
were not observed, but probably in the species of spider upon 
which the most complete observations were made two pairs of 
abdominal feet are involuted. 
hether the conversion of the abdominal feet into the lung 
book is to be regarded as ar involution of certain paired append- 
ages, as was suggested by Lankester on theoretical grounds, or 
as a portion of the abdomen over which an appendage cor- 
responding to the operculum of Limulus extends, could not be 
Positively determined. 
ere appears, from one series of sections, to be a swelling cor- 
responding in position to the operculum of Limulus just in ad- 
vance of the involuted abdominal appenda 
Another point of interest in arachnid embryology is the pres- 
ence of a fold in the blastoderm, surrounding the cephalic region 
of the embryo. Balfour described this fold as a groove. 
Pears, however, when studied by transverse and longitudinal sec- 
tions to be a fold of the blastoderm. At the anterior extremity 
of the fold its opposite sides unite over the median line of the 
embryo, so that the brain is partially invested by an outer 
1 or bag of epiblast formed by the united inner limbs of the 
°PPosite sides of the fold. 
, _J€ origin of this fold and the union of its opposite sides over 
the middle line of the embryo correspond to the amnion of in- 
Sects. The difference between the insect amnion and the spider 
amnion lies in the fact that in the former the union of the oppo- 
Site sides of the amniotic fold is in most cases complete through- 
eloped only in the head region of the embryo and coalesce 
at their anterior ends.—A. 7. Bruce, Ph.D. 
