634 ARACIINIDA. 



which divide it into such small fibres that it dries almost im- 

 mediately on coming in contact with the air. The spider has 

 the power of uniting these fibres into one or several threads, 

 according to the purpose for which they are to be used. The 

 thread commonly used for the web is composed of hundreds 

 of simple fibres, each spun through a separate tube. As the 

 thread runs from the body it is guided by the hind feet, which 

 hold it off from contact with surrounding objects, until the 

 desired point is reached, when a touch of the spinners fastens 

 it securely." (Emerton, American Naturalist, ii, p. 478.) 



The eggs are laid but once a year in June. The evolution 

 of the embryo begins immediately', and goes on with a rapidity 

 according with the temperature. The egg consists, as Herold 

 observed, simply of a vitelline membrane, but no chorion ; it 

 is perfectly homogeneous, and has no micropyle. The contents 

 are an emulsion of fatty globules suspended in a scanty 

 amount of liquid, which should not be confounded with the al- 

 bumen (or Avhite) of the eggs of vertebrates. No trace oH the 

 "germinative vesicle" has as j-et been traced in the eggs of 

 insects, though perhaps it has been overlooked from its trans- 

 parency. 



The first stages in the egg after the}^ are laid, are the follow- 

 ing : at the surface of the vitellus appear, here and there, 

 small, very clear and perfectly circular spots ; they are the 

 nucleus of the future blastoderm (primitive skin, from which 

 the organs of the embryo successively originate or "bud" 

 out). These nuclei act as centres of attraction on the mole- 

 cules of the vitellus for the formation of the cellules. The 

 unmodified vitellus diminishes in the same proportion as the 

 peripheric layer of granules increases. The granules multiply 

 rapidly, and soon the surface of the egg appears to be divided 

 into a certain number of areas, each of which is occupied in 

 the centre by a circular and transparent space surrounded with 

 small opake granules, which become less and less dense as Ave 

 go to the outer surface. These hexagonal cellules form an uni- 

 form layer over the entire surface of the egg ; it is the l)lasto- 

 derm. Up to this time the changes precisely accord with those 

 observed in the hexapodous insects. 



The next stage is the formation of ventral tubercles, the ru- 



