DESCRIPTION OP PLA.TE V— continued. 



should be more distinct : they are drawn as they appeared after tlie action of 

 chromic acid. 



_Pi(j. 7. — X 188. A portion of the periphery of a large follicle (about 2 

 mm. in diameter), showing the details of some structures already mentioned. 

 The reference figures have been already explained. The membrana granulosa 

 is relatively thin, and forms an even layer. The intermediate layer (i. I.) 

 is seen to form a coarse network, the origin of which from the cells of 

 the membrana granulosa is proved by actual continuity and by the presence of 

 nuclei at the nodal points. Nuclei are sometimes met with at a considerable 

 distance from the membrana granulosa. The central substance (c. s.) is 

 continuous with the strands of the intermediate layer. It forms a very 

 granular network, with thick irregular strands. These are the appearances 

 seen after the action of hardening reagents. The outermost cells of the 

 granulosa should be columnar. 



Fig. 8. — x 9. A transverse section of the ovary of Ornithorhynchus. 

 The whole organ was much contracted and altered, st. Stroma, very much 

 contracted, and of which it •was impossible to make out the true structure. 

 In the central part of the section there are a few small blood-vessels. The 

 ovary is a flattened oval body, and it is possible that blood-vessels are more 

 abundant in the central part of some other region than that seen in this 

 section, and it is likely that here they are much concealed by the contracted 

 state of the tissues. The follicles {F.) are very abundant all round the 

 periphery of the section, and the large ones project from the surface. There 

 is also a tendency towards constriction of the larger follicles, thus resembling 

 the arrangement in some other vertebrates where the constriction is carried so 

 far as to produce a stalked appearance. It is also probable that the constric- 

 tion is far more marked in a fresh ovary than in the one from which this section 

 is taken. The light spherical spaces inside the follicles represent the ova, 

 which are thus much larger than in other Mammalia, although the follicles are 

 far smaller than those of Phalangista. The line enclosing the light spaces 

 represents the zona pellucida (?) and the single-layered follicular epithelium. 

 Outside this is the basement membrane of the epithelium and an external 

 layer, which represents the fibrous tunic, c. I. Structures representing the 

 Corpora lutea. There seem to be different forms of these. In some few 

 cases I have found the follicular lumen apparently filled with yellowish cells, 

 and the fibrous tunic thickened. In other cases, evidently soon after the 

 escape of the ovum, the lumen was partially occupied by a yellow shrunken 

 clot, which had also stained the adjacent tissues. Most commonly, however, 

 the lumen seems to be obliterated by a gradual thickening of the tunic, in 

 which vessels can be detected. Some of those in the section are of this kind, 

 while others still show a slight trace of the lumen. It is of course probable 

 that some of the appearances mentioned (if not all) are difl"erent stages of one 

 history, but these apparently highly peculiar structures can only be satisfactorily 



