194 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
divisions, at least up to the formation of the planula, take place by 
mitosis. ‘lhe egg presents a rather marked polarity. Segmentation 
of the egg may follow nuclear division almost at once, though always 
slightly delayed, or nuclear proliferation may continue for some time 
before cytoplasmic division begins. A segmentation cavity usually 
occurs, though it may be represented by separate spaces between 
adjacent blastomeres. _A nearly typical blastula —a single layer of 
cells surrounding a large segmentation cavity — is sometimes produced, 
but this may be more or less modified, even to the complete obliteration 
of the cleavage cavity. In some cases no stage exactly comparable to 
a blastula can be discovered. ‘The solid mass of cells, the so-called 
‘morula,’ which sooner or later results, is to be regarded as the out- 
come of the process of germ-layer formation and not as the end of 
segmentation. ‘lhe germ layers are the result of a primary multipolar 
delamination, though the definitive ectoderm and entoderm are later 
more completely differentiated by a further division and rearrangement 
of the cells of the early germ layers. 
Double blastulae have been found, which are probably the result of 
an incomplete first cleavage. ‘The nuclei of early cleavage stages are 
usually single, those of blastulae usually consist of two vesicles. | Chro- 
mosomes arise independently but synchronously in the two parts of the 
double nuclei, and their distinctness may or may not continue through- 
out the mitosis. Daughter nuclei take the form of two apposed vesicles 
which, however, remain distinct through the entire resting period. 
When the germ layers have been definitely formed and are separated 
by the supporting membrane, double nuclei no longer appear. 
IV. Discussion and Historical Review. 
1. Growth of eqggs.— It is well known that in the Coelenterata, 
although the primordial germ cells are all alike, only a few of the many 
odcytes become mature, the rest serving as food for the survivors. 
Most authors have believed that the circumstances of favorable posi- 
tion and nourishment determine which odcytes become egg- and 
which food-cells. Brauer (’91>) and Wulfert (:02), however, believe 
that a differentiation occurs during the migration of the germ cells into 
the gonophore, though the latter maintains that this early differentia- 
tion is the result of favorable position and nourishment. Grénberg 
(98) and Miiller (:08), on the other hand, think the differentiation 
occurs in the oécytes. | 
