I'.X)/.] NATURAL SCIEXCES OF IMII L.VDFJ.IMII A. 533 



thoy have heon derived (I'l. XXXIX, fi<;s. 29, 31). At this time there 

 are three circles of four small cells each, lyiiiR on the surface of the e^g 

 and arranged around the animal pole (figs. 29, 31). These twelve cells 

 have been derived by three successive divisions of the large stem cells 

 of the first ciuartet, la^-*-l(^'\ The imier circle consists of the small 

 apical cells already described. The middle circle consists of the cells 

 ]^,i. 1.2. i_l^/i. 1.2.1 ^yi)i(^.i^ ^yei-g derived next and are the largest of these 

 twelve cells. The outer circle was derived at the last division described. 

 At the next division of these stem cells four more small cells are cut off 

 to the exterior. 



At the same time that the last division described is occurring, the 

 cells la^-'-l(/'---^ divide in a keotropic direction (fig. 28) into nearly 

 equal moieties. With the completion of the above divisions there are 

 forty-four cells in the first ciuartet, eleven in each quadrant. After a 

 short resting period the cells ia*-i-2i_i(/i-i-2-2.i^ w^hich have been so 

 activel)' dividing, prepare to divide again. This time they bud into the 

 interior of the egg four comparatively large cells, iai-i-2.2.i.2_2^i.i.2.2.i.3^ 

 the primitive ganglion cells. The outer smaller cells of this division, 

 j^,i. 1.2.2. i.i_j^^i. 1.2.2. 1.1^ form the fourth circle of small cells about the 

 animal pole. This is as far as I have been able to follow accurately the 

 divisions of these cells, and it is possible that the cells here designated 

 primitive ganglion cells may still bud one or two generations of ecto- 

 derm cells to the exterior. The four cells which are to form the gang- 

 lion divide repeatedly. Their individual divisions have not been 

 traced, but four strings of cells can be distinguished for some time, each 

 of which is the result of the subdivision of one of these primitive 

 ganglion cells. These ganglion cells lie at first just above the meso- 

 derm cells, 4(P-^-^ and 4cP-^-^, and it is extremely difficult to differentiate 

 the later divisions of these cells. 



In the divisions of this first quartet we have had a number of ex- 

 amples of the process already alluded to, in w^hich a very small cell is 

 budded to the exterior of the egg and partially covers the larger, deeper 

 Ijang moiety. Other examples of this same phenomenon will be 

 described in the history of the second quartet. The four sets of four 

 small cells each "which, beginning with the apical cells, are budded off 

 in rapid succession from the large stem cells of the first quartet ,la^-^- 

 Id}-^, are among the more striking examples of this phenomenon. These 

 sixteen cells (and possibly more) very nearly cover the aboral surface 

 of the egg, and by their further divisions the ectoderm of this region is 

 formed. Other cells of this first quartet show the same process more 

 or less strikingly. For instance, by the divisions of la'-'-ld--^ a very 



