610 HAROLD HEATH, 



2. FirstSomatolDlast. 



Attention has already been directed to the fact that in the early 

 stages of development no appreciable difference in the size of the second 

 quartette cells exists, and that the first cleavage is the same throughout ; 

 the same thing is true of the next division which occurs in the upper 

 of the two blastomeres producing the tip cells 2a^-^ etc. and post- 

 trochal cells 2ai-2. At the next division, however, close resemblances 

 in the lower cells cease for in the division of 2d^ a somatoblast is 

 formed while the division of 2a2 etc. produces post-trochal cells. 

 The significance of this cleavage is considered on page 641. 



The next somatoblast cleavage occurs in the tip cell {2d^-i) and 

 is precocious when compared with this division in the other quadrants 

 but otherwise it is exactly similar to them. (This cleavage has oc- 

 cured in Fig. 26.) The cell 2d2-i now divides. The spindle is leio- 

 tropic and produces a cell which occupies the same position that 

 2 a--i etc. does in the other quadrants. 



The stomatoblast 2d^-'^ now divides transversely into two cells 

 of equal size. This division consequently corresponds in direction to 

 that of the anterior secondary stomatoblast. 



The next cleavages occur in 2d^-^-^ and 2d^-^'^ which are the 

 products formed by the division of the tip cell; the cleavages are 

 usually dexiotropic and leiotropic respectively although rapid shifting 

 soon destroys this position of the cells (Fig. 36). In the other 

 quadrants the cells 2a^-^-^ and 2a^-^-2 etc. are secondary trocho- 

 blasts and never divide, but in the posterior quadrant they have 

 become secondarily modified and divide many times. 



The cell 2 d2.i.2 now divides in a horizontal direction (Fig. 36); 

 and a spindle soon appears in 2d2-i.i dividing the cell leiotropically 

 into two equal moieties. 



There are now ten cells in the ventral plate, and the number 

 becomes rapidly increased, but I have been unable to determine the 

 order in which they occur. 



The cells at this time and for a considerable period of their 

 development are relatively smaller than the neighboring third quartette 

 products and they are by no means so regularly arranged ; consequently 

 it is not a difficult task to determine the outlines of the somatoblast 

 up to the stage shown in Fig. 46. Beyond this point however they 

 assume the characters of the neighboring third quartette cells and it 

 becomes impossible to determine their outlines and consequently to 



