374 BEKTRAM G. SMITH 



occasionally be deceived in distinguishing the first from the second 

 cleavage furrow. One can hardly attribute to this method the 

 degree of accuracy inherent in the two preceding methods. 

 However, the results tend to confirm the conclusion that there is 

 no constant relation between the direction of first cleavage and the 

 median plane of the embryo. 



5. Instability of the micromeres. In studying the problem of 

 orientation of the early cleavage furrows it is necessary to bear 

 in mind the extensive shifting of the micromeres and consequent 

 torsion of cleavage furrows that takes place from the beginning 

 of second cleavage throughout the remaining early cleavage 

 stages (Smith, '12, II). The portions of the early cleavage 

 furrows that traverse the region of macromeres are relatively 

 stable, but, on account of the instability of the micromeres dur- 

 ing the early segmentation period, upper and lower portions 

 of a meridional cleavage furrow may come to he in different 

 directions. 



After keeping a living egg under constant observation during 

 the early cleavage stages and sketching the cleavage pattern at 

 frequent intervals, it is possible upon comparing these sketches 

 to trace the first two cleavage furrows through the region of 

 micromeres up to the sixth and sometimes the seventh generation 

 of blastomeres. This has been done in a number of instances, 

 and it has been found in every case that the path of a given cleav- 

 age furrow becomes very irregular. The most extensive shifting 

 occurs during the early stages, beginning with second cleavage; 

 later, as the micromeres become smaller, the distances involved in 

 these movements are not so great. 



D. The excentricity in the superficial cleavage pattern of the early 



blastula 



Since the study of conditions arising during cleavage necessi- 

 tates a rather precise designation of stages, we shall have occasion 

 to refer to these stages by the serial numbers adopted in an earher 

 paper (Smith, '12, II) devoted to the external development; the 

 entire segmentation period is divided into ten stages. 



