HARGITT: PENNARIA TIARELLA AND TUBULARIA CROCEA. 199 
formed, suggests the possibility that this body may contain substances 
necessary to the formation of the chromosomes, and if so, is not super- 
fluous matter nor a by-product. 
The opinion expressed by Bigelow (:07, p. 367) that “there is.... 
no conclusive evidence that the chief nucleolus in invertebrates ever 
normally contributes to the formation of the chromosomes of the first 
cleavage spindle,” is not borne out by conditions in Cunina (Stschel- 
kanowzew, :06), or in Tubularia crocea. And as regards other in- 
vertebrates, Jordan (:08%, :08>) has shown that the nucleolus of 
Echinaster fragments, and that from these fragments alone the chro- 
mosomes of the maturation spindle are formed; further, that in Asterias 
the nucleolar substance aids in forming the chromosomes. It seems 
to me that the claims of Rohde (:08) as to the close relationship exist- 
ing between all nucleoli and chromatin, which Bigelow says (p. 365) 
‘are, in the main, supported by the conditions in Gonionemus,” 
furnish presumptive evidence that the nucleolus may contribute to the 
chromosomes. If all nucleoli arise in the nucleus from chromatic 
microsomes, directly or indirectly, as Rohde claims, why may they not 
return to their original condition as chromatic bodies? ‘This appears 
to be exactly what happens in Tubularia, where the nucleolus dissolves 
or fragments entirely within the germinative vesicle, and the pieces 
become incorporated in the nuclear reticulum. As R. Hertwig (’98, 
p. 718) long ago said: “‘....in the dissolution the material of the 
nucleolus unites with the chromosomes,’ and in its new formation 
the nucleolus comes from the chromatin. Hertwig considers the 
nucleolar substance to act like a cement in uniting the chromatin into 
chromosomes. He says (p. 714) further, “....between plastin- and 
chromatin-nucleoli, indeed, no sharp boundary exists. ....between 
both forms of nucleoli transitions exist, and one can arise from the 
other.” Ruzicka (:06, p. 556) has summed up the matter thus: 
“The nucleolus can change itself into either nuclear substance, or 
cell body, or spindle, and can again reform from these conditions.”’ 
5. Cleavage.— Allen (:00) states that the first nuclei of segmenta- 
tion in ‘Tubularia crocea appear as small masses reorganized from a 
previously fragmented nucleus, and she found the earliest mitosis 
occurring when four nuclei were present. Hargitt (:04°, Pennaria) 
(:04*, Tubularia mesembryanthemum and other forms), Hickson 
(788, ’90, 94, Hydrocorallinae) and Hill (:05, Aleyonium) all speak of 
a nuclear fragmentation and a later reorganization of the fragments 
to form cleavage nuclei. In the specimens of Pennaria and Tubularia 
crocea which I have examined, there was no sign whatever in any egg 
