8 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



end until finally it forms a wide meridional valley encircling the 

 entire egg (Fig. 3, B). This valley gradually deepens dividing the 

 egg into two halves. Finally after about 5 minutes from the 

 commencement of the process the protoplasmic bridge connecting 

 the two halves snaps across and the egg is now completely divided 



Fig. 3. — Illustrating the process of segmentation of the egg of Amphioxus. 

 (After Hatschek, 1881.) 



The apical pole is above in each figure. The second polar body js seen in proximity to it. 



into two blastomeres, each of which assumes a spherical shape in 

 response to surface tension. The two blastomeres become slightly 

 flattened where they are in contact i.e. in the plane of the first 

 meridional furrow (Fig. 3, C). The future course of development 

 shows that this plane corresponds to the sagittal plane of the 

 embryo (Cerfontaine, 1906): in other words the two blastomeres 

 represent the right and left halves of the developing individual. 



