250 BULLETIN OF THE 



in the same plane as in former instances, the line indicating the first 

 plane of cleavage, which in them was unbroken, is bent at right angles 

 at the point where the secondary furrow has appeared. The second 

 cleavage furrow is at this time a little over .15 mm. long. The longer 

 diameter of the egg is .GO mm. ; the shorter, about .45 mm. 



The general appearance of the egg two minutes later than the last, or 

 at 1 h. 20 m. p. m. (fig. 17), although in most respects similar to it, has 

 several marked differences, the result of the progressive growth. One 

 of the most striking of these diff'erences is the still greater increase in 

 the amount of the deviation from a straight line which now separates 

 the lower end of the upper line from the upper end of the lower ver- 

 tical, both being the profile of the first cleavage plane, 1 cl. pi. The 

 length of the second cleavage furrow, se., has now increased to .25 mm., 

 its breadth remaining about the same, and in its sides are frequent pli- 

 cations running parallel with the first cleavage plane, much more 

 sharply defined than in any which has preceded it. Up to the present 

 time (fig. 17), twenty minutes after the first visible changes by which 

 we pass from an egg with two cells into one with four, the secondary 

 furrow has been limited in its extension. It now slowly deepens, and 

 at the same time grows along the surface of the ovum toward the 

 equator, although at 1 h. 20 m. it has not yet extended far enough to 

 reach the periphery of the egg as seen in profile. There is as yet no 

 indentation marking the limit of the second cleavage groove on the 

 equator of the egg. 



In the same egg five minutes later, at 1 h. 25m. p.m. (fig. 18), the 

 second furrow, se., is found extending across the whole hemisphere, and 

 is represented in the figure by the large horizontally placed furrow. 

 The size and depth of this indentation may be estimated by the depres- 

 sion at either extremity of this furrow. In profile it is seen to equal in 

 depth the radius of the egg. Like the primary groove, ^?r., this likewise 

 eventually extends almost through the egg, dividing it into two sym- 

 metrical hemispheres connected by an undivided " bridge." The walls 

 of the furrow, still grooved with cleavage folds, have not yet begun to 

 approximate. In this stage (fig. 18), although we seem to have four 

 segmentation sj)heres, the second plane of cleavage does not extend 

 more than two thirds across the diameter of the egg as seen from the 

 original surface of infolding. On the side of the egg away from the 

 observer, the 2-cell stage was slightly grooved by the second fuiTOw. 

 The sides of the second cleavage furrow have not yet begun to draw 

 together. At this time in the growth of the ovum the walls of the 



