6 



Papers from the Marine Biological Laboratory at Tortngas. 



body. Single, bilobed, and trilobed bodies can be explained in many in- 

 stances as portions of tetrads, the latter having been cut in various planes 

 or at different levels by the microtome knife. Several of the beaded chro- 

 matic threads have persisted, but they are frequently entirely wanting, 

 always few in number and very variable in morphological characters. 

 They may perhaps represent chains of smaller nucleolar fragments. 



I am inclined to believe that these very definite four-lobed chromatic 

 bodies are chromosomes, but I am aware that many objections may be 

 made to such an interpretation. The only decisive test of the matter is 

 lacking until the conduct of these elements is observed at the time of the 

 formation of the maturation spindle. But whether already chromosomes or 

 not, they must at least represent stages in the formation of the definitive 



3 4 



FIG. 3. Nucleus at still later stage. Nucleolar fragments separating. Nuclear reticulum, 



which remains pale-staining, shrunken away from wall. Cytoplasm still dark-staining 



and now appears as a mixture of fibrillar and alveolar types and contains abundant 



microsomes. < 1500. 

 FIG. 4. Nucleus of later stage, showing chromatin products of nucleus scattered through 



pale reticulm. X 1500. 



chromosomes. However, both cytological study of the eggs and macroscopical 

 examination of the gonads indicates that the eggs are close to maturation 

 and that the chromosomes have already final characters. For most echino- 

 derms thus far reported on, the somatic number of chromosomes is 36. If 

 these bodies are really chromosomes their number (reduced) here must be 

 several times as many. However, according to Boveri, Echinus inicro- 

 tubcrculatus has only 18 somatic chromosomes, and Tennent 1 reports a 



1 Tennent, D. H., 1907. Further studies on the parthenogenetic development of the 

 star-fish egg. Biol. Bull., Vol. 13, No. 6. 



