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 clearlv indicate, and later thev 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 ecpiator 

 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 exliibit ; 

 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 



