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 (:03) 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. 713) 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 

 ('88, '90, '94, Hydrocorallinae) and Hill (:05, Alcyonium) 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 



