358 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



there are fine lines which suggest a similar division. ^Vlien the contents are 

 compressed they fall into clumps like yolk segments. 



A smaller swelling contained a single body with a diameter of 0.058 mm. in 

 which no structure could be detected, and the distinction between cortical and 

 central mass was less obwous. On the other hand, in three bodies which were 

 contained in a larger swelling on the same pinnule, and which measured from 

 0.10 mm. to 0.12 mm. in diameter, the structure described was already indicated. 



Since none of the stained preparations exhibited any trace of nuclei, von 

 Graff concluded that they could not be ova. 



GENITAL CORD. 



Between the three extensions of the body cavity into the arms, the dorsal canal, 

 and the two ventral canals, there lies, in the thickened partition, a longitudinal 

 cavity, the genital canal, and in this lies the genital tube, which surrounds the 

 genital cord. From the arms the genital tube passes into the disk, where it runs 

 in a cavity communicating with the other divisions of the body cavity, and forms 

 an irregular pentagon lying between the ventral integument and the digestive tube. 



In the disk the genital canal has a lesser diameter than in the arms, measuring 

 only 0.02 mm., while the genital cord is 0.01 mm. thick; the latter is circular in 

 cross section. 



Ludwig described the lumen of the genital tube as a blood vessel, but Hamann 

 foimd no evidence for this view. 



The genital cord lies on the wall of the genital tube. It is composed of large 

 cells, which usually entirely fill the lumen of the tube. These are the primitive 

 germ cells from which in the pinnules arise the eggs and sperm. 



The cells within the genital cord are amoeboid, and often lie so thickly together 

 that their borders become unrecognizable. In Heliovietra glacialis they measure 

 0.08 mm. in diameter. They contain prominent large circular nuclei and have a 

 finely granulated cytoplasm. 



The nuclei of the cells of the outer epithelial layer of the genital tube lie so 

 closely together than the cytoplasm is almost entirely obscured. The tube wall 

 beneath this epithelial layer consists, according to Ludwig, of longitudinal fibers, 

 and at intervals there are ring-muscle fibers. Hamann also found a thin layer of 

 connective tissue. Within this is an epithelial lining, which extends over the 

 genital cord. 



The genital tube is attached to the wall of the genital canal by spindle- 

 shaped cells. 



The structure of the genital cord and of the genital tube is the same throughout 

 their course. 



No connection between the genital cords and the axial organ has been con- 

 clusively demonstrated. 



CEMENT SECEETINQ CELLS, 



In addition to the mucus-secreting cells of the ambulacral epitheliiun Schneider 

 found other cells, which he described as mucus-secreting, very sparingly distributed 

 in the epithelium of the ventral perisome. 



