FIRST PERIOD OF DEVELOPMENT. 45 



THE FIRST FURROW AND TWO-CELLED STAGE 



(Fios. 3, 4.) 



The first furrow makes itself primarily noticeable 

 as a depression on the upper pole of the egg in the 

 neighbourhood of the polar body. Then immediately 

 it grows round the whole circumference, and gradu- 

 ally begins to divide the egg in two parts. It is 

 however always deeper on the upper side, where it 

 first appeared (Fig. 3). Before the complete separa- 

 tion of the egg into two halves, 1 we have still a some- 

 what clear protoplasmic connexion, poor in yolk 

 granules. At last this also is separated into two, 

 and the two parts, which are distinguished from each 

 other by quite a sharp outline, take the spherical 

 form, and touch only at one single point. The for- 

 mation of the first furrow up to the complete division 

 into two segmentation spheres, occupies scarcely five 

 minutes. The two spherical portions now become flat 

 opposite each other in the plane of the first furrow, 

 so that their contact surface is a far greater one. 



The first segmentation plane is accordingly a meri- 

 dional one, and so far as can be observed divides the 



egg into two absolutely equal parts. The polar body 



1 Blastomeres. 



