SEGMENTATION 



11 



perpendicular to that of the first. It appears about three-quarters 

 of an hour after the latter and, like it, extends downwards and 

 inwards so that the egg becomes divided into four approximately 

 equal segments. 



The third furrow is latitudinal in position being situated 

 (Fig. 5, C) roughly about 20° above the equator. It extends 

 inwards and the egg is now converted into eight, blastomeres, four 

 micromeres towards the apical pole and four macromeres towards 

 the lower pole. 



Closer study of these first three cleavages in the case of the 

 Frog brings out a number of im- 

 portant points. It will be noticed 

 in Fig. 5 that the circular area of 

 the egg-surface which is free from 

 pigment is placed somewhat eccen- 

 trically so that at one edge it 

 approaches the equator of the egg 

 much more nearly than it does 

 at the opposite edge. It will be 

 noticed further that the egg as 

 judged by the distribution of 

 pigment is arranged symmetric- 

 ally about the plane of the first 

 furrow. This furrow seems to cor- 

 respond, under normal conditions, 

 with the sagittal plane of the 

 embryo, and therefore the two 

 hemispheres separated by the first 

 furrow correspond to the right and 

 left halves of the embryo. The 

 study of later stages will bring out 

 the fact that the point in the 

 boundary of the unpigmented 

 portion which lies nearest to the 

 equator marks what will become the posterior end of the embryo. 



From the time of appearance of the third furrow onwards wide 

 differences occur between different eggs. Occasionally one may be 

 found in , which matters proceed with diagrammatic regularity. 

 Two new meridional furrows appear intersecting the angle between 

 a and /S and like the latter they gradually extend downwards, 

 halving each of the existing blastomeres and thus giving rise to 

 sixteen blastomeres — in two tiers of eight, micromeres above, macro- 

 meres below. Then a latitudinal furrow appears dividing the 

 micromeres, and later a similar furrow dividing the macromeres ; so 

 that there are now four tiers of eight blastomeres each. 



Commonly however there is no such regularity either in the 

 arrangement or in the time of appearance of the furrows. The 

 meridional furrows in particular tend to be replaced by vertical 



E 



Fig. 5. 



Illustrating segmentation of Frog's 

 g. (After Schultze, 1899.) 



