446 MONTGOMERY. [Vol. XV. 



8. Lineus gesserensis (O. F. M.). 

 (Plate 24, Figs. 159-177.) 



Yolk. — The yolk first arises in the cytoplasm in the form of 

 irregular yolk balls, which are much smaller than in the other 

 nemerteans examined i^Yk. BL, Figs. 159, 160, 177); these 

 increase in number and size, the largest sometimes contain- 

 ing vacuoles. In the largest ovarial ova seen (though I had 

 only immature individuals of this species) yolk balls are no 

 longer present, but in their place smaller yolk globules, which 

 in all probability represent fragments of the earlier balls. The 

 yolk usually makes its first appearance in a zone of the cyto- 

 plasm, midway between the nucleus and the cell membrane, 

 which is characterized from the rest by a less dense structure 

 (Fig. 177). The extreme peripheral portion of the cytoplasm 

 retains its density longest, as is also the case in the other 

 species. The cytoplasm of the connective-tissue cells (Fig. 159), 

 from which the egg cells take their origin, stains very faintly, 

 while that of the young egg is dense and stains deeply. 



Nucleoli. — Only three worms out of eighteen sectioned con- 

 tained ovogenetic stages, and since in these individuals only the 

 earlier stages of this development were found, I am able to 

 describe only the younger stages of nucleolar formation. The 

 egg cell of this heteronemertean contains a single nucleolus ; 

 apparent exceptions will be considered later. 



In the smallest nuclei (Fig. 159) of the cell syncytium of the 

 gonads no nucleoli are to be seen ; we find nucleoli for the 

 first time in cells whose nuclei are a little larger and whose 

 cytoplasm commences to stain more intensely. These are the 

 earliest stages of the ovocytes. 



Now in these youngest germinal vesicles (Figs. 159, 161, 

 164, 166) the nucleolus is very frequently peripheral in position, 

 close to the inner surface of the nuclear membrane ; while in 

 the later stages (certain mitotic stages excluded) it is almost 

 invariably never in contact with the nuclear membrane. Fur- 

 ther, yolk balls first appear in the cytoplasm when the nucleus 

 contains a nucleolus. These facts, being considered together 

 with the fact that nucleoli are absent in the nuclei of the con- 



