From Krohn's description it is clear that he saw, for 

 the first time, in the cavity of echinus division that 

 accumulation of cells, which was called by the latest 

 embryologists the primary mesenchyma. He correctly determined 

 the fate of these cells in larval development. This discovery 

 of Krohn's was so thoroughly forgotten that Mechnikov, who was 

 well acquainted with the old embryological literature, 

 attributed it to Selenka, whose work was published exactly 

 thirty years later after Krohn's monograph. 65 



"The development of Echinus micro tub eraulatus ," Mechnicov 

 wrote, "was studied by Selenka. The first differentiation of 

 the embryonic layers begins with the bulging of the lower 

 cells of the blastoderm. Until the beginning of their 

 protrusion they form a number of wandering cells, which later 

 develop into the cutis and are considered mesodermal or 

 mesenchymal cells. "66 



Krohn observed that after nearly thirty hours after 

 fertilization the following important changes take place in 

 the larva of Echinus lividus . In the center of the blunt pole 

 a small hole-like deepening appears. The blunt pole widens and 

 thickens; the hole in its center becomes deeper and wider, 

 so that the body of the larva becomes similar in form of an 

 apple. The more enlarged hole "formed as a result of that," 

 Krohn wrote , 



caused the skin in this area gradually to protrude 

 (EINSACKT ODER EINWARTSSTULPT) into the cavity 

 of the body. The protrusion (EINSACKUNG) submerged 

 deeper in the body cavity and extended into a canal 

 which finally reached the walls of the body cavity 

 toward the blunt pole. The sac which appeared by this 

 means stretched through the body cavity (Figure 45, 

 C, h, k3 and is the rudiment of the digestive cavity. 

 The edge or circumference of the primary hole becomes 

 a leading opening in the canal, which is the anus. In 



65. E. Selenka, "Keimblatter und Organanlage bei Echinicen," 

 ZTS. WISS. ZOOL.,, 33 (1879) . 



66. I. I. Mechnikov, "Vergleichend-embryologische Studien. 



3: Uber die Gastrula einiger Metazoen," ZTS. WISS. ZOOL., 

 37 (1882) , pp. 286 - 313. 



580 



