HARGITT: PENNARIA TIARELLA AND TUBULARIA CROCEA. 201 
gonophore; but there seems also to be a tendency to an abbreviation 
in this stage of the development. ‘This abbreviation is most evident 
when the cleavage cavity is greatly reduced or lacking, in which case 
cleavage and germ-layer formation may not be sharply separated from 
each other, as Wulfert (:02) also found in Gonothyraea. 
By a division of the cells of the blastula, the cleavage cavity becomes 
entirely filled with cells. ‘This happens by the process designated by 
Metschnikoff (’86) as primary delamination of the multipolar sort. 
The same thing was found to occur in ‘Tubularia mesembryanthemum 
by Brauer (’91 ) and by other workers on other Hydromedusae. 
Since a blastula stage does occur, this part of the development clearly 
belongs to the germ-layer formation, the inner cells representing pri- 
mary entoderm and those at the surface primary ectoderm; a true 
morula, therefore, does not occur. The definitive ectoderm and 
entoderm are formed by a further division, specialization, and read- 
justment of the primary ectoderm and entoderm cells, and the eventual 
formation of a supporting lamella between them. 
This interpretation is not at all inconsistent with the syncytial 
conditions described by Hickson (790, ’94), Hargitt (:04*, :04°), and 
Brooks and Rittenhouse (:07). Indeed, from the figures of the last 
two authors it appears that the syncytium represents the so-called 
morula, and that an earlier primary delamination has occurred. 
Their figures also show between the cells spaces, more or less connected, 
which are indications of a reduced segmentation cavity. ‘The very 
great delay in the formation of definite cells in Eudendrium (Hargitt, 
:04>) and the Hydrocorallinae (Hickson, 1. c.) is due, as these authors 
state, to special conditions of development, viz: the presence of yolk, 
and development within a very narrow cavity, from which the embryo 
must later migrate. 
6. Double nuclet.— Multivesicular nuclei have been found in 
many animals, chiefly in the re-formation of the daughter nuclei after 
mitosis. Often each chromosome occupies at first a separate vesicle, 
but these commonly fuse into a single vesicle, which is characteristic 
of the resting condition. Such nuclei have been found in Hydro- 
medusae by Metschnikoff (’82), Wulfert (:02), and Hargitt (:04°). 
Distinctly double nuclei, similar to those found in the cells of the 
blastula and later stages in Tubularia, have been less often described. 
In Coelenterata only Conklin (:08) found sometimes in Linerges in 
the first division such double vesicles, which he thinks represent the 
halves of egg and sperm nuclei. Hacker (’95) and Riickert (’95) 
were the first to describe such nuclei, finding them in Copepoda. 
