i2 Papers from the Marine Biological Laboratory at Tortugas. 



SUMMARY AND CONCLUSION. 



In EcJiinastcr crassispina the chromosomes are derived exclusively from 

 llu- nucleolus. In Ophiocoma piunila the chromosomes arise exclusively 

 from the nuclear reticulum. The germinal vesicle of both species con- 

 tains a chromatic nucleolus. There are here two extreme types. Asterias 

 forhcsii furnishes an intermediate type in that here the chromosomes assume 

 a more or less intimate connection with the nucleolus prior to maturation 

 and receive substance therefrom. Eggs of Toxopneustes variegatus, parthe- 

 nogenetically developed after treatment with MgCL, according to Wilson 

 yield the two extremes in different sets, there being in one case present a 

 plusmosome and in the other a chromatic nucleolus. It appears that differ- 

 ent forms of echinoderms differ in the matter of the origin of the prematura- 

 tion chromosomes. In some species the chromosomes arise from a chroma- 

 tin-nucleolus, in others from a chromatic reticulum, and in still others in part 

 from one source and in part from the other. Again, the eggs of different 

 forms appear to differ in that some have only a chromatin-nucleolus, without 

 distinct plastin ground-substance, resting in an achromatic nuclear reticulum 

 (Ecliinaster) ; others possess both chromatin-nucleolus and plasmosome as 

 well as a chromatic nuclear reticulum (Ophiocoma) ; and still others possess 

 a double nucleolus ( chromatin nucleolus and plastin ground-substance), with 

 the chromosome complex gathered in a mass in the achromatic reticulum 

 (Asterias}. 



The chromosomes thus arise inconstantly in different species from any 

 part of the germinal vesicle that contains the chromatin material, and this 

 may be either nucleolus, nuclear reticulum, or both. The function of the 

 germinal spot then appears, in part at least, to be that of a storehouse of 

 material which is to contribute to the formation of the chromosomes. What 

 chromatin is not so employed is resorbed by the cytoplasm, probably return- 

 ing to the elements from which it was elaborated and serving as a food mate- 

 rial. There appears nothing here to support or confirm the theory of the 

 individuality of the chromosomes, but rather much to arouse suspicion 

 awarding the theory. But one may take refuge in the idea of " centers of 

 chromosome activity " as suggested by Davis, 1 and so the chromatin may 

 perhaps be regarded as merely the garb for the determinants of inheritance, 

 and the characters that arise in their manifold variations as the result of a 

 quantitative as well as a qualitative distribution of chromatin. 



' Davis. I',. M., 1905. Studies on the plant cell. Am. Nat., Vol. 39. 



