NOTES ON THE EMBRYOLOGY OP LIMULUS. 
537 
face) there grows from the dorsal wall of the lung a blind pro- 
jection (Auslaufer), by which the leaf-formation of the interior 
of the pulmonary cavity is begun. The external opening is at 
this time evidently smaller. The walls of the embryonic lungs 
are lined with cylindrical epithelium, which secretes a thin 
cuticula. On the outer upper surface of the lung occur here 
and there some cell masses, which belong to the middle layer 
Bertkau (72, pp. 211, 212), speaking of the increase in size 
of the lungs of Lycosa, says : “Mit dem Wachsthum der Spinne 
wachst auch der Luftsack und zwar starker als das Stigma, so 
das seine Spitze bald weit von demselben entfernt ist. Der 
erste Anlage eines Fachers zeigt sich in Auftreibungen 
des Bodens des Luftsackes, von diesen die jedesmalig jiingste 
dicht neben der nachst alteren entsteht und durch Intussus- 
ception neuen Bildungsmaterials wachst. 
Both of these accounts, as far as they go, agree with the 
development as it occurs in Limulus, and the addition of new 
lamellae, as described and figured by Bertkau, is exactly paral- 
leled by that occurring in Limulus, both in position and in 
manner, if we accept Lankester's views or those given below. 
It, however, seems to me hardly necessary to invoke the aid 
of “ parabranchial stigmata ” in order to derive the internal or- 
gans of the Arachnids from the gills of Limulus. The change 
in the needs of the animal owing to its assumption of a terre- 
strial in place of an aquatic life would seem to be sufficient to 
account for a part. Such a change in habit would necessitate 
a protection for the respiratory organs, and this would be best 
secured by an outgrowth of the surrounding parts, so that 
finally appendages and gills would be enclosed in pits, the open- 
ings of which would then grow smaller. The greatest difficulty 
with this whole homology, as carried out by Lankester, is that 
this opening must become completely closed, and another one 
developed, that the side which at first was exposed to water 
and then to air should subsequently be bathed with blood, 
while the portion which originally contained the vascular fluid 
should finally become opened to the air. Although I believe 
that there is a genetic relationship between the respiratory 
