249 



emm 



{a) 



gives rise to mesoblast. This last cell soon gives evidence of an in- 

 dividuality by dividing a considerable time before the others. This is 

 the first actual bilateral division in the egg, although it is foreshad- 

 owed by an almost bi- 

 lateral division of the [b] 

 X cell (2d) and by divi- 

 sions in the posterior 

 arms of the cross. The 

 first real bilateral di- 

 vision occurs at an 

 unusually late period, 

 as the embryo then 

 consists of over 86 cells 

 (Fig. 2, J-). The ento- 

 blast plate arises from 

 eleven cells, namely, 

 the four macromeres, 

 the four cells of the 



fifth quartet, and three 



cells of the fourth 



quartet. After most of 



this plate has invagin- 



ated, the M. or ento- 



mesoblast cells, closely 



pressed together, also 



sink in and lie under 



the posterior lip of the 



blastopore. These cells 



are not now of exactly 



the same size, the right 



being slightly larger. 



Immediately after pas- 

 Fig 2 View from lower pole of a stage of over 130 cells {A). The basal macro- 



meres and the fifth quartet have sunken in so far that only a small part appears on 



TesuSce. Their älations to the other quartets have also J^ecome changed hr^^^^^^ 



shiftines The ectomesoblast from the a quadrant (ewm) is still at the suitace. ine 



cells afting from the third quartet are connected by arrows and the groups are also 



''''''''^tX^:^^^7^-^^^l to and little below the prototroch of a slightly older stag^^). 

 Ectomesoblast dotted entomesoblast (coelomesoblast) marked out by heavy Imes. 5a-5d 

 fl^?th Quartet (entomferes); M-M entomesoblast divided bilaterally; re rudimentary cells 

 nthrsetid quartet in ^. e.nr, ectomesoblast right arising from the c quadrant; emi 

 ectomesobSt leftarising from the ä quadrant. em,n, ectomesoblast median, arising from 

 the a quadrant. 



enfM 



{a) 



