picture of fissures on the ovum surface; also Baumgartner, 

 who incompletely described the phenomenon. The merit of 

 final discovery of the secret of ovum division undoubtedly 

 belongs to Baer (107) . The study of the developed ova of 

 brown and green frogs led him to the conclusion that 

 fissures, which are seen on the surface of the yolk, repre- 

 sent nothing more than the boundaries of the divisions 

 which are manifested by the whole embryonic sphere. Baer 

 gave to embryological terminology names for the fissures of 

 division. He named the fissures connecting the poles of 

 the ovum, the planes of which pass through the axis of 

 the ovum, the meridional fissures; the fissure crossing the 

 axis of the ovum at a right angle, he called equatorial if 

 it divided the axis approximately in half and parallel if 

 it was situated near one of the poles. In addition to the 

 accurate description of division, Baer thought he saw 

 non-existent phenomena in the frog ovum. Thus he referred 

 to an orifice which seemed to be present in the region of 

 the dark pole and which he thought led through a canal into 

 the deeper cavity and remained there even after the dis- 

 appearance of the embryonic vesicle. This description came, 

 evidently, from the penetration by the spermatozoa, which 

 leave behind the dark trace that Baer erroneously took for 

 a canal . 



The phenomenon which Baer called transformation is 

 externally detected by the development of the first merid- 

 ional fissure. He described it in extreme detail, since 

 he clearly recognized its importance. Five hours after 

 oviposition, the first meridional fissure forms beginning 

 from the dark pole and it gradually moves from there along 

 a spherical arch in the direction of the light pole, where 

 its ends are united. This process of fissure formation, 

 Baer asserted is not continuous, but takes place in separate 

 stages as if the movement of the fissure were overcoming 

 some resistance. The essence of this first transformation 

 in the developing ovum is that the "yolk sphere is divided 

 into two hemispheres or, more accurately, into two spheres, 

 which become attached to each other" (p. 487). 



After the closure of the first fissure, according to 

 Baer, "apparent quiescence begins. However it only seems 

 to be, because the cleavage is imperceptibly distributed 

 from the inside surface" (pp. 487-488). 



421 



