hargitt: pennaria tiarella and tubularia crocea. 189 



variable in their relation to the spadix of the gonophore. Reference 

 has already been made to the difficulty of finding maturation spindles 

 and polar cells, and to the close similarity between the latter and 

 degenerating oocytes. These conditions contribute to the difficulty 

 of determining the poles of the egg. The relation of the germinative 

 vesicle to the spadix is shown in Figures 55, 56 (Plate 7), which repre- 

 sent nearly mature eggs in which the germinative vesicle lies at the 

 periphery and probably marks the point where the polar cells would 

 have been formed. In all eggs which I have examined the germinative 

 vesicle, just previous to polar-cell formation, is on the side of the egg 

 which is opposite the spadix, and this holds good for the position of 

 such polar cells as I have seen ; but no more definite relation between 

 the spadix and the animal pole of the egg could be determined. As 

 will become clear during the description of the later stages, these facts 

 are in harmony with cleavage conditions. For instance, during the 

 early cleavages the nuclei are always on the side of the egg opposite 

 the spadix, and cytoplasmic division seems to start in this region. 

 We are justified, then, in assuming a more or less definite polarity 

 for the egg, as Allen (:00) earlier pointed out, and as others have 

 observed in the eggs of coelenterates. It seems to me, however, that 

 a comparison of the germinative vesicles in growing eggs warrants 

 the conclusion that their position in relation to the spadix is not defi- 

 nitely fixed, and therefore that the exact position of the pole is variable. 



a. Early Cleavage. — Figures 57a and o7b show the nuclei of an 

 egg just after the first cleavage. Interzonal filaments are present and 

 the reorganization of the chromatic substance has resulted in a double 

 vesicle in each case. From conditions found in other nuclei, it is very 

 probable that these would have fused into a single vesicle. Figure 57a 

 shows one polar cell, which probably still occupies the animal pole 

 of the egg. 



Figures 58a and 58b show the second nuclear division completed, 

 from which it is clear that the division of the two nuclei has been 

 almost perfectly synchronous. Figure 58c, a portion of figure 58b 

 more highly magnified, shows the appearance of the interzonal body, 

 of the remains of the interzonal filaments, of the nucleus reorganized 

 into a single vesicle and of the second cleavage furrow at the beginning 

 of its formation. That the division of the cytoplasm lags behind that 

 of the nucleus, is evident from an inspection of Figures 58a and 58b, 

 the first cytoplasmic division having proceeded only a short distance 

 into the egg, even after the second nuclear division. This condition 

 is the usual one, for in no case in the early cleavage stages have I found 



