JEXXINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 35 



The sequence of cleavage is the same as in previous divisions, but 

 with some modifications. In any given layer of cells, the order is D, 

 C, B, A; a repetition of the sequence established at the third cleavage. 

 In any given quadrant, the order of cleavage varies with the relative 

 size of the cells. In the quadrant D the order is (beginning with the 

 ventral cell) 1, 2, 4, 3, and this is also the order of relative size of the 

 cells, beginning with the largest. In the other quadrants the order of 

 cleavage is 4, 2, 1, 3, and this again is the order of comparative size 

 beginning with the largest, except that 2 and 1 are so nearly of a size 

 that it is difficult to say from observation that either is the larger. 



The fifth cleavage is accompanied, as a result of the changes in form 

 of the cells during karyokinesis, by a partial enclosure of the ventral 

 cell of quadrant D {d^-^) by the other cells. 



The above account of the fifth cleavage is based upon an examination 

 of twenty-five eggs, taken from different individuals and showing difier- 

 ent phases of the division ; i. e. each egg contained more than fifteen 

 and less than thirty-two cells. 



Sixth Cleavage. 



The first division belonging to the sixth cleavage, that of d'^-^, takes 

 place coincidently with the last division of the fifth cleavage, that of a^-^. 

 There is thus no resting period between the two cleavages. Xeverthe- 

 less, there is a sufficiently well characterized stage of thirty-one or 

 thirty-two cells, just as the cleavage of d^-^ occurs, and it will be well 

 to describe the egg in this condition as a basis for an account of the 

 sixth cleavage. 



Figure 43 (Plate 5) shows the anterior surface just before this stage 

 is attained ; Figure 47 (Plate 6) shows nearly the same surface after the 

 fifth cleavage is finished. The posterior sui'face is shown in Figures 46 

 (Plate 6), 53, and .54 (Plate 7) ; the animal pole, in Figure 4-5 (Plate 

 6). Figure 48 shows a sagittal section, while a transverse section of a 

 stage just later (looking toward the animal pole) is given in Figure .52. 



The principal axis of the e^<^ still coincides with its long axis, the 

 animal pole lying at or near the micromere end, the vegetative pole at 

 the macromere end (Fig. 48). 



The e^s, now consists of (1) a single large cell, d^^^, embedded within 

 the other cells and appearing on the surface at the ventral end only 

 (Plate 5, Fig. 38), and (2) of thirty-one smaller cells, partly surround- 

 ing the larger cell c?®*\ One of these smaller cells, rf®*^, is a minute 

 vesicle embedded between the cells a^-^, h^'^, and d^-^ (Figs. 38 and 42). 



