606 HAROLD HEATH, 



division and not only is it similar in appearance but their destiny is 

 the same, both being post-trochal cells. 



Considering the cells at this time we find each group nearly 

 bilaterally symmetrical with reference to a principal axis meridian 

 passing through it (Fig. 25). The blastomeres are grouped in the 

 form of a Y the stem being 2a2. 2 etc., the limbs 5«-?.-2 and 2 a'^- 1 

 etc., while supported in the angle between these latter cells is the 

 tip cell 2ai-^ etc. For the only time in their history the tip cells 

 now divide. The cleavage is dexiotropic and produces two cells which 

 enter the functional velum and following Mead are termed the 

 secondary trochoblasts (Fig. 27, 30). 



The spindles introducing the next cleavage are perfectly meridional 

 and the completed division results in two products in three quadrants, 

 the lower smaller ones being situated between the parent cells and 

 the fourth quartette. These cells (2 a ^•^•■2 etc.) enter into the form- 

 ation of the stomadaeum and are the secondary stomatoblasts. 



The cleavage following this occurs simultaneously in 5 a ^-2 and 

 2a2i etc., being leiotropic and dexiotropic respectively (Fig. 30). I 

 believe that these cells do not divide again, certainly not up to the 

 point where they form an almost continuous row about the embryo 

 (Fig. 47). 



As a usual thing before the above cleavage is complete another 

 occurs in 2a'^-2.i etc. producing two cells of almost equal size lying 

 one above the other (Fig. 30, 33). 



The next division occurs in the anterior secondary stomatoblast 

 2 b 2. 2. 2^ The spindle occupies the plane represented by the arrow 

 in Fig. 39. The innermost cell is generally about two thirds the size 

 of the parent cell and lies directly upon the fourth quartette cell 

 which is usually in the process of division at this time. 



The cells 2 a 2- 2.1.1 etc. now divide into a right and left half of 

 equal size, and almost immediately this occurs in 2a2-2.i2 etc. 

 (Figs. 39, 42, 44). The division of the right and left secondary stomato- 

 blasts occurs at the same time as the above and the characteristics 

 of the cleavage are best described by the drawing (Fig. 39). The 

 approach of the ectoderm cells towards the vegetative pole causes 

 these cells to be crowded into the embryo. As a usual thing the 

 tertiary stomatoblasts encroach upon them to such an extent that at 

 the time of their division very little of their surface is exposed. It 

 is to be noticed that this cleavage is bilateral. 



