294 



INVERTEBRATA 



CHAP. 



221 



uppermost cells of the egg, Iq^, divide into larger daughters, Iq^^ 



below and very small 

 daughters, lq^\ above, 

 which occupy the ex- 

 treme animal pole of 

 the egg. 



The egg up till now 

 is perfectly radially sym- 

 metrical, and no differ- 

 .]^22i ence whatever can be 

 2ia detected between its 

 different quadrants in 

 this respect. With the 

 sixth cleavage, however, 

 leading to the formation 

 of the 64-cell stage, this 

 radial symmetry is lost, 

 and is replaced by a 

 bilaterally symmetrical 

 arrangement. 



The divisions which 

 constitute this cleavage 

 are as follows. The cells 

 Iq" divide in regular 

 spiral order into cells 

 Iq"^, which correspond 

 to the apical cells of 

 Polygordius, and into 

 Iq"^ the peripheral ros- 

 ette cells, which corre- 

 spond to the Amielidan 

 cross in the larva of 

 Polygordius. The • cells 

 Iq^^ also divide spirally 

 into daughters of equal 

 size, and give rise to the 

 so-called MoUuscan 

 cross, which at this stage 

 has curved arms. The 

 upper daughters, Iq^^^, 

 form what are called the 

 basal cells of the cross ; 

 the lower daughters, 

 lqi22^ constitute the 

 intermediate cells of 

 the cross, but the tips of 

 the four arms are constituted by the four tip cells, 2qi'-, which 

 belong to the second quartette of micromeres. The cells Iq^^ 



Fig. 227. — Two stages in the development of the upper 

 hemisphere of the embryo of Patella coerulea viewed 

 from above. The primary trochoblasls are nu- 

 shaded. The cells belonging to the second quartette 

 are dotted, those forming the Molluscan cross are 

 ruled with horizontal lines, whilst those forming 

 the Annelidan cross are covered with small circles. 

 The apical cells are unshaded. (After Wilson, some- 

 what altered.) 



A, stage in which the cells forming the Molhiscan cross 

 are undergoing their first division. B, stage in which the cells 

 forming the Molluscan cross are dividing for the second time, 

 and in which the cells forming the Annelidan- ci'oss have 

 appeared. 



