DEVELOPMENT OF THE OVARIES AND OVA. 193 



From the labours of Schenk (101) we know that in the first in- 

 stance the whole pleuro-peritoneal fissure is lined on its inner surface 

 by columnar cells proceeding from the separated lateral parts of the 

 middle germinal layer. Schenk is of opinion that this layer of cells, 

 corresponding to the cutaneous and interstitial fibrous layer, is applied 

 simply and exclusively to the production of the later appearing epi- 

 thelium of the peritoneum. Gotte has recently expressed himself in 

 favour of this view in regard to the Batrachia. These columnar cells 

 soon disappear over the greater part of the peritoneal cavity, being 

 replaced by pale quite flat elements, the median angle alone corre- 

 sponding to the middle plate of Remak, and subsequently to the 

 Wolffian body which appears at this spot, retaining its columnar epi- 

 thelial covering. 



About the fourth day a considerable thickening of the above- 

 mentioned epithelium occurs both on the median and on the 

 lateral side of the Wolffian body ; the thickening on the median 

 side is the first rudiment of the ovary, the lateral goes to form 

 the subsequently appearing tube called Miiller's duct. 



In the male embryo the thickening of the epithelium appears first 

 in the situation of the sperm gland, but disappears again about the 

 eighth or ninth day ; whilst it continues to increase in the female. 



In a short time a small outgrowth, rich in cells, projects 

 from the interstitial tissue of the Wolffian body beneath the 

 epithelial thickening (see fig. 196). The thickened epithe- 

 lium investing this outgrowth gradually forms the rudiment 

 of the Graafian follicles and ova, and of the subsequently 

 appearing epithelium of the ovary, whilst the outgrowth 

 itself is destined to furnish the vascular stroma of the 

 ovary. 



As soon even as the fourth or fifth day of incubation the in- 

 teresting observation may be made in the embryoes of fowls, 

 that some amongst the epithelial cells have become conspicuous 

 by their round form, their size, and the large nucleus they 

 contain (see fig. 196, o). We may conclude from the regular 

 arrangement of these structures, and the constancy of their 

 position, that they represent the youngest primordial ova, 

 which thus, even during embryonic life, are formed by a simple 



VOL. II. O 



