180 bulletin: museum of comparative zoology. 



connect the nucleolus with the nuclear reticulum, the chromatin 

 appearing in the reticulum in scattered masses, chieflv at the nodes or 

 close to the nuclear membrane. In preparation for the last oogonial 

 division, the chromatin collects into larger masses, which may become 

 strand like, but do not seem to form a definite spireme. The concen- 

 tration continues and the chromosomes in the form of a closely packed 

 mass constitute the equatorial plate (Plate 4, Fig. 29, o'gd^.). Mean- 

 while, the nucleolus has disappeared. The last oogonial spindle has 

 no asters, neither could centrosomes be demonstrated, though prob- 

 ably they are present. During the division the chromosomes remain 

 so closely massed that their individuality is entirely hidden. The com- 

 pact cytoplasm, even before division is complete, may become more 

 distinctly granular and less deeply staining (Fig. 29, ogd".). Some of 

 the oogonia do not undergo this final division with their sister cells, 

 and probably serve to start the next generation of eggs, which develop 

 when the first have formed embryos. 



2. Oocytes. — The final oogonial division results in the primary 

 oocytes. The chromatin of each oocyte remains for some time in a 

 compact mass (Fig. 29, ocy^.); but after a time it becomes more 

 open. Meanwhile a nucleolus, which selects the plasma stains, has 

 made its appearance (Plate 5, Fig. 30), and the chromatin mass has 

 opened out into small irregular masses, evidently pieces of the original 

 chromosomes. At this time a differentiation of the oocytes occurs ; 

 some of them at once become young egg cells, while others do not 

 develop further. The latter are present in larger numbers, most of 

 them furnishing food for the growing eggs. In these the chromatin 

 becomes arranged in irregular masses along a delicate linin reticulum, 

 which is limited mostly to the outer part of the nucleus (Fig. 34a). 

 While tliese oocytes are perhaps not destined to become egg cells, they 

 do not show degenerative changes (unless they are being, or are about 

 to be absorbed), and hence under favorable conditions they may 

 retain the capacity to form egg cells. In the oocytes which begin to 

 grow at once the chromatin forms a definite spireme, at first so closely 

 massed as to show little sign of its thread-like character (Fig. 30). 

 As it becomes more open the spireme is easily made out, though I have 

 not been able to determine whether a single thread or several threads 

 are present. Figure 29 shows two oocytes (o'cy^.) already of con- 

 siderable size, which, from their relative position and from the presence 

 of interzonal filaments, are clearly sister cells. In these the spireme 

 is well shown and each contains a single large nucleolus, though the 

 one belonging to the left-hand cell does not appear in this section. 



