284 GEO. A. THIEL AND HAL DOWNEY 



boundary. Similar conditions were described by Laguesse, and 

 Mietens noted that cells from the epithelium of the mesentery 

 (Bufo) join the mesenchyme in the early stages. 



These findings are in accordance with the observations of Mol- 

 lier, who found peritoneal epithelium and mesenchyme forming a 

 distinct and continuous network over the early embryonic liver. 

 He states that ''Das Peritoneum ist fast iiberall keine glatte, 

 scharf begrenste Zellhaut, sondern nunmt im Verein mit tiefer 

 gelegenen Stellen an der Bildung eines Netzwerkes Anteil, in dem 

 alle diese Zellen mit protoplasmatischen Auslaufern zusammen- 

 hangen und grossere und kleinere Maschenraume abgrenzen." 



The process of cell division is very active in embryos of this 

 size, but the proliferative activity is not confined to any particu- 

 lar region of the visceral mesoderm; it is rather uniformly dis- 

 tributed throughout the entire peritoneum. Janosik held that 

 the mesothelium was very active in the formation of free cells 

 in the region of the early spleen. However, at no stage of em- 

 bryonic life, has the writer found cell division more active in the 

 mesothelium of the mesentery than in any other part of the peri- 

 toneal epithelium. But the mesothelium in the region of the 

 splenic rudmient does give off some cells to the underlying mesen- 

 chyme, as man}" mitotic figures are seen in the epithelium with 

 their long axes perpendicular to its free surface. One of the 

 daughter cells of such divisions is consequently crowded beneath 

 the epithelium and becomes part of the mesenchymatous net- 

 work. However, equally as many mitotic figures are seen with 

 their long axes parallel with the surface of the epithelium, thus 

 merely adding more cells to the peritoneal tissue. 



In a 7.5-mm. pig the peritoneum gives evidence of becoming a 

 more distinct structure, but still many areas can be found where 

 cells are being displaced from the peritoneum and are crowded 

 into the mesenchyme. Figure 1 is taken from the peritoneum 

 over the region of the early splenic rudiment. There is very 

 little difference between the cells of the coelomic epithelium and 

 those of the mesenchyme. The cytoplasm of the former is 

 shghtly more basophilic, but the nuclei of the two tissues are 

 identical. In both the epithelium and the mesenchyme, the 



