OVARIAN OVUM OF MARSUPIALIA AND MONOTREMATA. 127 



secondary yolk. These finely-granular layers, and the sphe- 

 roids within, together with the external investments, have a 

 very obvious resemblance to Waldeyer^s figure of the follicle 

 of a fowl (fig. 19J-, p. 184, vol. ii. of the English translation 

 of Strieker). In fig. 11, the nucleus is situated in the layer 

 of spheroids ; it is probably on its way towards the circum- 

 ference. 



The corpora lutea are very remarkable. The most important 

 change that takes place in their formation seems to be the 

 thickening of the tunica fibrosa, which thickening continues 

 until the cavity is almost, or quite, obliterated. This change 

 appears to begin early, and sometimes to be the only change, 

 but of this I cannot speak with certainty. In the immensely- 

 thickened layer blood-vessels are common. Such structures, 

 with walls of all degrees of thickness, were very common in- 

 deed (see fig. 1). Sometimes the lumen appeared empty, and 

 sometimes it appeared to contain a shrunken basement-mem- 

 brane with a few cells, perhaps of the follicular epithelium. 

 In other and far rarer instances I observed masses of yellow 

 cells within the thickened walls. It may be that this last is 

 the initial, the former the ultimate stage, and that thus the 

 corpora lutea are not very remarkable in this animal; but 

 the objection to this is, that the masses of yellow cells are 

 comparatively rare, and that they do not form large and 

 important masses, and further, that the former changes seem 

 to be taking place at an early date, as far as one can judge. 

 In a few cases the lumen contained a yellow shrunken clot 

 which had also stained the adjacent tissues. This whole ques- 

 tion needs further elucidation. 



The condition of the ovaries of Echidna was such that I 

 could make out nothing, except the important fact that the 

 follicular epithelium and zona peilucida are similar to Ornith- 

 orhynchus. I was enabled to ascertain this from an examina- 

 tion of scattered segments, such as I have described. 



These results as to the relation of the ovum within the ovary 

 are very important, being characters that persist over important 

 sections of the animal kingdom. If the conclusion that I have 



