l6o BERTRAM G. SMITH. 



originates from the center of the first furrow, and seldom reaches 

 the lower pole, but extending obliquely in the lower hemisphere 

 joins the second furrow at some distance from the pole (see plate, 

 Fig. \6B, line b ; Fig. 17 B, lines b, g and e\ Fig. iS-B, lines b, 

 i and c, etc.). Hence the third cleavage furrow is intermediate 

 between a true meridional cleavage and a latitudinal one, approach- 

 ing more nearly the meridional type. This will be made clearer 

 by supposing the point of origin of the third cleavage furrow to 

 be shifted along the first further from the animal pole ; and its 

 point of junction with the second to be shifted further from the 

 lower pole ; the third groove will then become latitudinal. On 

 the other hand, if the two ends were shifted in the reverse direc- 

 tions, the cleavage lines would become truly meridional. Varia- 

 tions from the customary mode of cleavage tend to confirm the 

 view that it is an intermediate or transitional form ; while some 

 third cleavage furrows originate at the center of the animal pole, 

 giving a true meridional cleavage, others come in latitudinally. 

 (See Figs. 9-12.) 



D. The FonrtJi Cleavage Furrow (Figs. 13-15). This comes 

 in about 3 hours later --a briefer interval than that preceding 

 the appearance of the third cleavage furrow. In its typical con- 

 dition it is parallel to the equator, but close to the animal pole, 

 so that the division is very unequal. Irregularities in this cleavage 

 are very numerous. 



E. Later Segmentation Stages (see plate, Figs. 16-24). The 

 later divisions occur with increasing rapidity, but with such irregu- 

 larity that no definite arrangement of cleavage furrows can be 

 made out. Cleavage proceeds rapidly near the animal pole, 

 much more slowly in the equatorial region, and is greatly re- 

 tarded in the lower hemisphere. At some time during the 

 second day after the egg is laid the first cleavage furrow reaches 

 the lower pole. At this time the upper hemisphere is cut up 

 into a considerable number of cells. The macromeres continue 

 to be very much larger than the micromeres. While so far as 

 can be determined from a surface study of the egg, the cleavage 

 is undoubtedly holoblastic, a strong tendency toward the mero- 

 blastic condition is evident. 



