hargitt: pennaria tiarella and tubularia crocea. 203 



it "is not at all in evidence" — is also true for Tubularia, and this 

 seems to be fatal to the view of the autonomy of the sperm chromo- 

 somes and egg chromosomes in these cases. On the other hand, 

 since in Tubularia these double nuclei occur in blastulae and in the 

 forming germ layers when a rapid and continuous division and differ- 

 entiation of cells is taking place, may it not have some connection 

 with an intense nuclear activity? R. Hertwig (:08), referring to the 

 "mulberry-formed" and lobed nuclei found in histology, development 

 and pathology of metazoa, says, a comparison with similar nuclei 

 in Protozoa shows that these nuclear forms, in the Protozoa, corre- 

 spond to critical stages in the cell life, brought about by a strongly 

 active functioning. Such an activity seems to be indicated in Tubu- 

 laria at these stages, and the double nuclei may have the same relation 

 to this cell activity. However, the symmetry, regularity and distinct- 

 ness of the halves of the nuclei, and even of the definitive chromosomes, 

 seems entirely inexplicab'e and superfluous on this view. 



The similarity in appearance between the resting stages of double 

 nuclei and stages in amitosis is rather close, but the history of these 

 nuclei shows that amitosis does not occur. Divisions follow one 

 another rapidly, and the interzonal filaments of the mitotic figure 

 remain present for so long a time that the history of a nucleus for 

 several generations is apparent almost at a glance. The result is that 

 nearly all such double nuclei, as well as those apparently single, in 

 the blastula and during the formation of the germ layers, can be proven 

 to have been formed by mitosis, and many show an approaching, or 

 actually beginning, subsequent mitosis. Since the same direct descent 

 of nuclei in cleavage stages up to the blastula is nearly as certain, and, 

 as we have seen, always leads to mitosis, we can claim with reasonable 

 certainty, that in Tubularia crocea nuclear division occurs by mitosis 

 from the first cleavage to the formation of the definitive ectoderm and 

 entoderm, at least. 



V. Summary. 



A. Pennaria tiarella. 



1. Nucleolus. — The oocyte nucleolus is a plasmatic body, which 

 dissolves within the germinative vesicle before the nuclear membrane 

 is ruptured. The linin network of the germinative vesicle extends to 

 the nucleolus, so that an exchange of substances may possibly occur 

 between the chromatin and the nucleolus. 



2. Polar Cells. — Polar cells seem to be formed just before, or at 



