196 BARBARA HEFFNER. 



they have only quite recently been discovered by (.ulick in tin- 

 nematode Heterakis (Boveri, '10), by Baltzer in the ediinoid-. 

 and by Guyer ('090 and b) in vertebrates. 



Observations on metakinesis stages (Baltzer '090, Plate 

 XXXVII., Fig. 9) have shown that the hook-shape of certain 

 chromosomes in Strongylocentrotus is due to the fact that the 

 spindle fiber from each pole is attached at a point about one 

 third of the length of the chromosome from one end, so that a 

 shorter and a longer arm are formed. In the horseshoe-shaped 

 chromosomes in Echinus the fiber is attached about half way 

 between the two ends, so that the two arms are nearly of the 

 same length (ibid., Fig. 10). As for the origin of the hook- 

 shaped and horseshoe-shaped heterochromosomes, observations 



Strongylocentrotus c 

 on cross-fertilized eggs, , . < - , and on multipolar 



mitoses of Strongylocentrotus have shown that they come from 

 the female pronucleus, the corresponding pair in the male being 

 rod-shaped. Apparently the female has an unequal pair of 

 heterochromosomes, one hook- or horseshoe-shaped, the other 

 rod-shaped; while the male has a corresponding equal pair of 

 rod-shaped chromosomes. It may be mentioned that this is the 

 reverse of what is found in insects, but as in most insects the 

 female nucleus must obtain more chromatin than the male 



nucleus. 



MATERIAL AND METHODS. 



My observations were made upon eggs of Toxopneustes and 

 Arbacia, collected and preserved by Professor Tennent. As 

 preserving fluid either picro-acetic or sublimate-acetic was used. 

 Sections of 5 /* thickness were stained with Heidenhain's iron 

 haematoxylin, except in a few cases mentioned later. 



The figures are all drawn with Abbe's drawing camera, Zeiss 

 oil immersion 2 mm. apochr. objective, oc. 12, enlarged to twice 

 or four time the original diameter and reduced one half. 



ARBACIA PUNCTULATA. 



The eggs of this species are quite unfavorable for detailed 

 cytological studies. Not only are the chromosomes very small 

 but the cytoplasm of the egg is filled \\ith pigment granules so 



