40 



ARACHXIDA. 



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

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

 Scorpio. 



The method of cleavage of the Araneid 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. 



Fio. 21.— Sections through the egg of Theridium mmmlatum 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 JE), 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] 



