94 Embryonic Development of Ovary and Testis of Mammalt 



III. OBSERVATIONS UPON SUCCESSIVE STAGES OF DEVELOPlSrENT 



IN THE PIG. 



0.7 cm. Embryo. — The mesonepliros is covered by a more or less dis- 

 tinct peritoneal layer, which is not clearly differentiated from the 

 stroma, except in the dorsal and lateral portions, but becomes increas- 

 ingly distinct on the medio-ventral surface, where the genital ridge 

 later takes its origin. The transition is, however, a very gradual one 

 and the differences slight. There is a rather loose vascular mesen- 

 chyme tissue that fills in the space between the peritoneum on the one 

 hand and the Malpighian corpuscles and mesonephric tubules on the 

 other. 



The cells of both the peritoneum and 

 underlying mesenchyme do not have de- 

 finite boundaries, appearing in this, and 

 in later stages as well, to form a 

 continuous protoplasmic network, to 

 which the nuclei give character by their 

 more or less definite arrangement. A 

 region of the peritoneum extending from 

 the base of the mesentery one-third the 

 distance to the Wolffian duct is of par- 

 ticular importance, since it is the rudi- 

 ment from which the genital ridge takes 

 its origin (Text Eig. 2 and Plate I, Eig. 

 3). A point about opposite the twen- 

 tieth glomerulus marks the boundary 

 between the future sex gland and the rete. 

 In the region of the genital ridge 

 (Plate I, Eig. 3), as defined above, 

 the greater part of the nuclei are of various shapes and sizes and stain 

 rather deeply with ha3matoxylin. The nuclei of the peritoneum are 

 closely packed together and are usually elongated by mutual pres- 

 sure. They rest upon a loose felt-like basement membrane, which is 

 formed by the interlacing of numerous slender branching protoplasmic 

 fibrils given off by both the peritoneal and stroma cells. The peri- 

 toneal origin of the stroma is clearly indicated at many points where 

 mutual pressure of the peritoneal cells is crowding them through the 

 basement membrane, which has, in fact, disappeared at such spots 

 as a result of this process. The positions and angles of inclination 

 of the columnar nuclei give satisfactory evidence on this point. The 

 presence of numerous mitotic figures in the peritoneum indicates a 



Fig. 2. Transverse section of me- 

 sonephros and associated structures. 

 Pig- embryo 1.4 cm. Jengtti. d. a., 

 dorsal aorta: </., glomerulus; %. m., 

 mesentery of the intestine ; Ji., liver; 

 m. /., mesentei-ic fundament ; 8. {/., 

 sex gland; u. t., uriniferous tubule; 

 W. a.. Wolffian duct, x 26. 



