1895] ; Embryology. 289 
These results are opposed to those of O. Hertwig who found that 
exposure to 0° C injured the eggs so much that they would not de- 
velop. 
` A Problematical Structure in a Mammal.—Dr. A. W. 
Weysse® in a very careful and well oe Doel of | some pow | 
blastodermic vesicles of the pig, finds a remarka 
arching over the germinal disk. The cavity beneath this “ sf is 
never closed, and is eventually obliterated by the fusion of the “ bridge” 
cells with the subjacent ectoderm. 
The author thinks this structure has no homology in any thing as 
yet known among mammals, but may, perhaps, be compared to the 
dorsal growth in Amphioxus that forms the medullary canal, since 
they both agree in time of formation and in relations to the neurenteric 
canal and neuropore! 
Development of Scyphomedusz.—Ida H. Hyde has pub- 
lished the results of a most careful examination of the cleavage, gas- 
trulation and the formation of the scyphistoma stage in several meduse. 
The paper is illustrated by more than one hundred very careful and 
true figures and bids fair to clear up some much disputed points upon 
which the most noted investigators have held different views. 
The material was obtained at Annisquam, Mass., at Eastport, Maine 
and from Johns Hopkins Marine Station in Jamaica, and was studied 
at Bryn Mawr, Woods Hole and Heidelberg. - 
The embryology of Aurelia marginalis from Jamaica had never 
before been studied ; that of A. fluvidula and Cyanea arctica had been, 
imperfectly. 
he gastrulation of A. marginalis differs from that of all known 
Scyphomeduse in that it is a process of delamination, that is, the 
blastula becomes converted into a closed two-layered larva by the 
division of some cells in such a way that their inner ends form an inner 
layer. 
In A. flavidula the germ layers are formed in two different ways : 
eggs from Eastport, Maine, gave rise to the entoderm by a process of 
delamination combined with inwandering of cells from various parts 
of the blastula wall; eggs from Annisquam formed invaginate gastru- 
las. 
In C. artica the gastrulation is a modified invagination in which some 
cells break loose from the pole that is invaginating ; there is, however, 
* Proc. Amer. Acad. Arts & Sci., XXX, 1894. 
