The reduction is brought about simply by the segmentation of the spirem thread into half the number of segments instead of the normal number.
Segmentation takes place during its passage down the uterus.
The segmentation of the prosoma and the form of the appendages bear a homoplastic similarity to the head, pro-, meso-, and meta-thorax of a Hexapod with mandibles, maxillary palps and three pairs of walking legs; while the opistho io i e d c b o a S' S" 2 I VT V S IV III II I Opisthosoma Prosoma FIG.
Segmentation in these is very unequal, and results in the formation of small cells called micromeres and large cells called megameres, as in fig.
But in a few European and North American species, and in a great many inhabitants of the tropics, the egg is large and a considerable portion of it persists for a long time as a yolk-sac. Although the segmentation is always complete, it is very irregular in these types, some of which make a distinct approach to the meroblastic egg.