40 



ARACHNIDA. 



the region in which the germ-band appears later, and may possibly 

 correspond with the germ-disc from Avhich the blastoderm spreads in 

 Scorpio. 



The method of cleavage of the Araueid egg agrees closely with that of 

 Crustacean eggs classed under type II. (Vol. ii. , p. 109). If, as appears probable 

 to us, a cleavage-cavity does not occur in all Araneid eggs, the centre in some 

 cases remaining filled with an unsegmented mass of yolk, these latter cases- 

 would probably be referable to that type which was described, in connection 

 with the Crustacea, as total cleavage with subsequent transition to superficial 

 cleavage. 



Fig. 21. — Sections through the egg of Thcridium naculatum in different stages of cleavage and 

 blastoderm-formation (after Morin). bl, blastoderm; d, yolk ; dp, yolk-pyramids ; dz, yolk- 

 cells ; fh, cleavage-cavity ; p, periplasm. 



[The blastomeres are, for the most part, flattened (Fig. 21 E), but 

 at one spot the cells become spherical and multiply rapidly, conse- 

 quently a large accumulation of blastoderm-cells, several deep, forms 

 at this spot. By reflected light this spot appears as a round whitish 

 area. Shortly after this a second thickening, and consequently a 

 second white area, appears. These two thickenings herald the 

 formation of the germ-disc] 



