HARGITT: PENNARIA TIARELLA AND TUBULARIA CROCEA. 193 
least, to the fact that the line of division between the halves was parallel 
to the plane of the section. ‘There is no difference in this regard 
between the nuclei of cells destined to form ectoderm, and those which 
are to form entoderm. 
Each resting nucleus typically consists of two vesicles (Plate 9, Figs. 
71-73), each of which contains one or more nucleoli and a network in 
which the chromatin is arranged. ‘That the halves are entirely distinct, 
is sometimes very plainly shown (Fig. 75), especially when the vesicles 
are so oriented that the plane of their contact is perpendicular to the 
plane of the section. ‘These nuclei have arisen by mitosis, as the 
interzonal filaments of Figure 73 clearly indicate, and later they divide 
by mitosis (Fig. 74). The chromosomes arise independently, but 
synchronously, in the halves (Figs. 75, 76), though it is often impossible 
to determine whether they remain separately grouped after the disso- 
lution of the nuclear membrane. As soon as they enter the equator 
of the spindle, they become so massed together that their individuality 
is hidden, and polar views of the spindle in this stage often show the 
chromosomes in a single mass. On the other hand, sections of the 
spindle when seen in polar view sometimes show the chromosomes 
still arranged in two groups (Figs. 79a—-79c); while in these cases the 
individual chromosomes are not distinct, their arrangement in two 
groups is unmistakable. ‘These figures are especially selected to 
show the double character which the chromosomes sometimes exhibit; 
but many other spindles in similar stages when seen in polar view do 
not show a double grouping of the chromosomes. Hence, the double 
nature of the nucleus may not always manifest itself in the spindle 
stage. In the reorganization of the chromatin to form the daughter 
nuclei, however, two vesicles were almost invariably found (Fig. 77), 
and these, by an increase in size, assume the condition characteristic 
of the resting stage (Figs. 71-73). 
When the cells of the germ layers are beginning to take their defini- 
tive positions and arrangement their nuclei usually appear single, 
but two nucleoli are generally present. By the time the ectoderm 
cells have attained the size and arrangement found in the planula and 
are separated from the entoderm cells by a supporting membrane, 
the nuclei of both layers have entirely lost the evidence of their double 
nature, each nucleus being spherical, and containing a single nucleolus. 
Double nuclei are entirely lacking in the planula and actinula and were 
never found in the cells of the gonophore or in the primordial germ cells. 
To recapitulate: segmentation is total, and may be nearly equal or 
decidedly unequal, and it may be more or less irregular. All cell 
