The following stage in transformation consists of the 

 appearance of a second meridional fissure, appearing six to 

 seven hours after fertilization. It goes also from the dark 

 region to the light, and its plane is situated under and at 

 right angle to the first. The process leads to "cleavage 

 of the sphere into four quarters," as can be confirmed by 

 cutting the compact ovum. 



In the third stage of transformation, the equatorial 

 fissure appears and division into eight spherical parts 

 attached to each other begins, followed by two parallel 

 fissures (above and below the equator) producing sixteen 

 yolk masses. Further division continues until there are 

 sixty- four. All previous divisions were performed in 

 vertical (meridional) , or horizontal (equational or par- 

 allel) planes. 



Baer hinted at the difference in the rhythms of division 

 of the upper and lower halves of the ovum. "It must also be 

 noticed," he pointed out, "that the first and also the 

 second meridional fissures in the light region move slower 

 and fit in less deeply" (p. 489). However, in his drawings 

 the inequality of division of the animal and vegetative 

 halves of the ovum is clearly represented. Later, the 

 division of yolk regions into central and peripheral regions 

 takes place. At this time there are already so many fissure - 

 separated regions, that the surface seems shagreen (leathery), 

 and after a little time when the total number of the separated 

 yolk masses is approximately 3,000, the surface is very finely 

 granulated, resembling grains of sand. Finally, the divided 

 parts of the ovum become microscopic, their surface seems to 

 be smooth, and a great number (many thousands) of yolk 

 aggregations can be found in one section. In the following 

 stage of development, the formation of the embryo (KEIM) 

 takes place, according to Baer, and the demarcation of the 

 embryo begins. 



His description shows that Baer could clearly follow 

 the process of complete division of the ovum and establish 

 its various stages. In the following pages of his outstanding 

 work, "General Observations of the Mechanism of Division," 

 he attempted to reveal the regularity of division processes 

 and to understand their purpose. First, he objected to formal 



422 



