1 8 Vertebrate Embryology 



completely separated into two blastomeres of 

 approximately equal size, which, at first, tend 

 to become spherical in shape, but which are 

 soon flattened against each other to form 

 hemispheres. Before the formation of this 

 first cleavage plane, the segmentation nucleus 

 has divided into two equal parts, one of which 

 is found in each of the two blastomeres. In 

 dividing, the nucleus passes through a com- 

 plicated series of changes known as karyoki- 

 nesis, for a description of which the reader is 

 referred to more extensive text-books. The 

 first cleavage plane corresponds to the medio- 

 longitudinal (sagittal) plane of the future frog : 

 this, however, is not true of all animals. 



After a short resting period, the second 

 cleavage plane is formed, preceded, as in the 

 former and as in all subsequent cases, by the 

 division of the nucleus of each blastomere. 

 The second plane is also a vertical one begin- 

 ning in the dark pole, and is at right angles to 

 the first plane. The egg now consists of four 

 more or less equal blastomeres (Fig. i, C). 



The third plane is normally a horizontal 

 one, at right angles to the first two, but not in 

 the equatorial plane of the egg, so that the egg 

 is divided into eight cells (Fig. i, /?), four 



