102 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



3. Nucleolus. 



The nucleolus was left at the time when it had just assumed a nearly 

 spherical shape, but was still attached to the membrane of the germinal 

 vesicle. It progressively assumes a more central position, but remains 

 attached to the membrane for a considerable period by means of a stalk 

 (Plate 3, Figs. 20-22). This stalk is at first quite large, but be- 

 comes smaller as the nucleolus increases in size, and finally disappears 

 altogether, never being encountered during the later stages. It cannot 

 be seen in all the young vesicles, and would of course be invisible except 

 in a profile view ; but from the generality of its occurrence I believe 

 it is a normal structure serving to attach the nucleolus to the mem- 

 brane from which it has been derived. Floderus (pp. 214-215) has 

 described nucleoli with similar projections attached to them, and 

 believes that he has in them stages in the formation of " nebennucleoli ," 

 but the fact that the largest stalks are attached to the smallest nucleoli, 

 which is also brought out by his figures (Taf. X., Figs. 19, 20, 21), 

 has no significance according to this view. 



When first differentiated, the nucleolus appears homogeneous with 

 all the stains used; soon, however, this condition changes and a cen- 

 tral medullary mass and an enclosing membrane can be distinguished. 

 In the most favorable haematoxylin preparations (Plate 6, Fig. 51) the 

 membrane takes a deep stain, the medulla (med.) appears granular and 

 more refractive, and outside of the membrane a faintly staining cortex 

 (ctx.) can be distinguished. With methyl green and acid fuchsin the 

 medulla is stained red, and is surrounded by a blue rim that is prob- 

 ably composed of both the membrane and the cortex seen in the 

 hoematoxylin preparations. The next changes consist in simple 

 growth, the medulla, surrounded by its membrane, coming to occupy 

 an excentric position within the cortex, which is increasing more 

 rapidly than the medulla (Plate 6, Figs. 52, 53). Next we have 

 developed within the medulla, and often very close to its membrane, a 

 variable number of very highly refractive bodies (cp. ref.). At the 

 same time the rest of the medulla becomes less refractive and more 

 finely granular (Plate 6, Figs. 54, 55). With the methyl green 

 and acid fuchsin combination, the cortex and medulla are with diffi- 

 culty distinguishable, both taking a pale bluish green stain; but the 

 refractive bodies are colored a very bright pure green. They are 

 therefore chromatin and apparently pure nucleic acid. At this stage, 



