OVAKIAN OVUM OF MAESUPIALIA AND MONOTREMATA. 125 



a layer, condensed from the general substance of the organ, 

 upon the outside of large follicles, but it was impossible to in- 

 vestigate its minute structure. Blood-vessels were not common 

 or large in the central tissue of my sections (taken transversely 

 across the middle of the organ). It is certain that they were 

 much contracted, and perhaps often obliterated. But while I 

 was able to make very little out of the general structure of the 

 organ, I was more fortunate with the follicles, in which some 

 most important points were still recognisable. On first exa- 

 mining some of the largest follicles (about 1 mm. in diameter, 

 but probably contracted), I found the ovum much shrunk, and 

 generally disintegrated, bat between it and the follicular walls 

 there were a number of short segments of the same curvature 

 as the follicle (or rather less). Further examination showed 

 that these segments consisted of two layers of about equal 

 thickness, the inner being homogeneous, and the outer evi- 

 dently composed of a single layer of cutical cells. Careful 

 search over a large number of sections enabled me to prove 

 that the outer layer was the follicular epithelium — a single 

 layer during the whole time that the ovum remains 

 in the follicle. Therefore Ornithorhynchus follows the lower 

 type, and never gains the characteristic mammalian granulosa 

 with many layers of cells. In many follicles of the largest size 

 1 found the single-layered epithelium still adherent to the wall 

 over a large part of the circumference. The epithelium formed 

 a very persistent layer, and in many cases it remained con- 

 tinuous when separated from the follicle by the contracting ovum. 

 When thus separated, it was possible to observe the layer from 

 the surface, and it then appeared as a mosaic of cells. The 

 inner part of the segments is an investment of the ovum — pro- 

 bably the zona pellucida. This layer is generally closely 

 adherent to the epithelium, and thus both were shattered when 

 the ovum contracted. It is very common to find a third finelv- 

 granular layer of about the same thickness as the others on 

 the inside of the segments. Those few fortunate sections 

 which showed these layers in situ proved that this third laver 

 is the external part of the ovum (see fig. 12). Under high 



