VII 



ANNELIDA 



137 



In the quadrant D the divisions at this stage are similar, but 

 2d222 and Sd^^i do not divide until the Trochophore stage is reached. It 

 follows that the so-called larval mesoderm is formed from the second 

 quartette in three of the four quadrants of the egg. 



We now pass to the consideration of the third quartette, and we 

 would remind our readers that this quartette is radially ^ituated, 

 whereas the one we have just been considering was inter- radial ; and 



2Z'=2' 



3c2W 2cf2 



Fig. 103 {contimied). — Three stages in the segmentation of the lower or vegetative surface 

 of the egg of Polygordius. 



A, stage of about 76 cells ; B, stage of about 112 oeUs ; C, later stage in which a mass of rapidly 

 dividing cells at the lower pole is sharply distinguished from an outer zone of clear cells. The heavy 

 black line surrounds the cells which later will take part in the process of invagination and the 

 formation of the lips of the blastopore. The cells belonging to the second quartette are dotted, 

 those belonging to the third quartette are marked by vertical lines. The cells belonging to the 

 fonrth quartette are marked by little circles, those belonging to the iifth quartette by horizontal 

 lines. The residual raacronieres. and those belonging to the first quartette, are left white. The names 

 of the cells which form the larval mesoderm are surrounded by circles. Of (2a^ip). 



further, as in eggs with spiral cleavage in general, the second and 

 third quartettes of micromeres come to lie about the same parallel of 

 latitude, so to speak, on the globe represented by the whole egg, since 

 the quadrants of one quartette occupy the gaps between the 

 quadrants of the other. 



Taking then the quadrant A first (and what applies to A is true also 

 of B) we find that 3a divides into Sa^ and 3a^ ; and each of these now 

 divides into an anterior and posterior cell, i.e. 3a^^ 3a^P, oa?-^ and 

 3a^p, respectively. Of these the first two 3ai^ and 3a^P, remain 



