BROOKS AND RITTENHOUSE: ON TURRITOPSIS. 
453 
The formation of the germinal layers in Turritopsis is different 
from that which has generally been described for the development of 
hydromedusae. In the majority of forms previously studied the 
differentiation took place either by delamination or by cellular ingres- 
sion, unipolar or multipolar. These methods have been well de¬ 
scribed and figured by Metschnikoff. 
In Aglaura and Rhopalonema there is found according to Metsch¬ 
nikoff a solid so called morula stage destitute of cleavage cavity, the 
superficial cells of which are converted into the ectodermal layer, 
while those within represent the endoderm. Here the two layers are 
formed directly without the formation of a syncytial structure. 
In Eudendrium and Pennaria according to Hargitt’s description, 
a condition somewhat similar to that of Turritopsis is found. He 
says: “Indeed in both Eudendrium and Pennaria, not to mention 
other cases, cleavage would seem to result primarily in the formation 
of a more or less characteristic syncytium, the subsequent development 
of the germ layers taking place by a gradual differentiation of the 
syncytial elements, first and naturally the ectoderm, and later, often 
very much later, the endoderm/’ 
The syncytial character in Turritopsis is acquired under favorable 
conditions, when the embryo is about six hours old, at the time that the 
irregular mass of segmenting cells is metamorphosed into the oval 
embryo. I am inclined to think that the formation of the syncytium 
and the change of shape of the developing embryo are connected 
phenomena. The length of time during which this condition lasts 
is evidently comparatively short, for soon cilia develop and the larva 
begins to swim; but meanwhile the peripheral region of the syncytium 
has been transformed into a distinct layer of ectoderm cells, separated 
from the inner mass of tissue, still structureless in character, by the 
development of the mesogloea. 
From the fact that a syncytium or plasmodium-like structure is 
formed, it is impossible to localize any of the blastomeres of the seg¬ 
menting egg which will form special parts of the future embryo. Even 
those cells which are at the surface at the completion of segmentation 
cannot be regarded as primitive ectoderm, for in the breaking down 
of the cell boundaries, the formation of the syncytium, and the recasting 
of the cells, \t is quite impossible to say what changes of the proto¬ 
plasm may take place. 
