DESCRIPTION OF PLATE V— continued. 



cases only showing the fibrous tunic, in others the membrana granulosa ; while 

 in one instance the intermediate layer is also cut through and appears as a 

 central light area surrounded by the darker membrana granulosa, g.f. Groups 

 of small follicles, in some cases containing a larger follicle with the primary 

 single-layered epithelium, advanced into the many-layered membrana granulosa. 

 These small follicles form an irregular cortical layer of Schron, external to 

 this is the albuginea and germinal epithelium. Isolated follicles in various 

 stages are also scattered through tiie cortical part of the ovary. When these 

 are pressed by the rapid growth of a large follicle, they give way and become 

 drawn out in the direction of the circumference of the latter (this is well seen 

 in Pig. 4). c. I. Corpus luteum, filled with yellowish cells, among which the 

 external tissues intrude, carrying blood-vessels with them. V. P. The very 

 vascular central part of the organ (zona vasculosa). g. c. Lobules of granular 

 cells, often arranged in elongated masses. These are doubtless the remains of 

 the Wolfiian body. Although especially present towards the central part of 

 the organ, scattered lobules and masses also occur in the cortical part (Pig. 4). 



The next five figures (2 — 6) show the gradual development of a follicle, the 

 earliest stage being shown in the small follicles of Pig. 4. All the figures 

 are magnified fifty diameters, so that the relative sizes express a true ratio. 



Pig. 2. — x 50. A small follicle, with the included ovum, m. g. Membrana 

 granulosa, with its external basement membrane. The shape of the cells is 

 very difficult to determine : sometimes they appear polygonal, sometimes 

 flattened, and sometimes there is an indefinite appearance of columnar struc- 

 ture in the external layer. The nuclei stain deeply. The indefinite shape of 

 the cells is doubtless due to their extreme flexibility. Subsequent investiera- 

 tiou has shown that the outer cells are columnar, the others polygonal. It is 

 perfectly clear that there is no regular layer of columnar cells, with a radiate 

 arrangement, round the ovum. In fact the layer next the ovum often seems to be 

 composed of rather flattened cells. Here again is an important point in whicii 

 the Marsupial follicle does not follow a very characteristic feature of the higher 

 Mammalia, o. The ovum itself, the reference mark being placed in the 

 vitellus. The zona pellucida (?) is excessively thin, and I could detect no 

 trace of another layer. The substance of the vitellus appears as an extremely 

 fine network (in this specimen hardened with chromic acid), which does not 

 stain readily, n. The subcentral nucleus (germinal vesicle). This structure 

 sometimes stains very slightly, or not at all (as shown in this figure) ; while 

 in other cases it stains deeply (Pig. 3). After the methods employed in 

 hardening, it shows a coarse network. It possesses a distinct limiting mem- 

 brane. There is a distinct spherical nucleolus (germinal spot), which is 

 clearly defined and generally very dark. 



Fig. 3. — X 50. The reference marks are the same as those of the last fig. 

 The nucleus in this case-stains deeply, and it is placed at the circumference of 

 the ovum. 



