566 CLASS IX. 
separated from the yolk, afterwards lies between two lateral portions 
of the yolk, which, by transverse indentations, change into ccecal 
sacs placed in pairs. These sacs, in connexion with the intestinal 
canal, are the rudiments of the biliary vessels and of the liver. The 
yolk changes into the liver, or the so-called adipose body. The 
limbs arise as conical appendages placed under the ventral surface, 
with the extremities turned downwards towards each other. On 
the dorsal surface of the yolk is seen a streak running longitudi- 
nally as the rudiment of the heart, that is at first without vessels. 
The nervous system, in its central parts, is formed at a still earlier 
period, and the cerebral ganglion is in the beginning proportionally 
much larger than it is afterwards}. 
We have already said that the scorpions are viviparous. With 
the egg-laying spiders, the egg, under the changes of development, 
slowly loses its previous form, and almost assumes that of a spider, 
indicating all the external parts of the inclosed animal. At length 
the shell bursts on the thorax, and the spider, first with the head, 
and afterwards with the thorax, comes to view; then follows the 
abdomen, to which however the egg-membrane, like a scale, con- 
tinues attached for a time; then come the feelers and feet?. The 
young spider, through whose integument the granules of the yolk 
may be clearly distinguished, is not yet in a state to weave a web 
and catch its prey; for the spinning organs are still concealed be- 
neath the common integument. After the lapse of a week, or, in 
some species, a longer time, during which the spider takes no food, 
it casts its skin for the first time, and is, as it were, born for the 
second time. The young spiders now quit, on some mild day in 
May or June, the web in which the mother had hidden her eggs; 
they allow themselves to fall on the ground by a thread, and begin 
at once to weave their nets, or in some other way, according to the 
instinct of their kind, to watch for small insects corresponding to 
their age and powers. 
1 On the development of spiders see M. Heroup De Generatione Aranearum in Ovo, 
Marburgi, 1824, fol. cum tab. eneis: Dr WirticH Observationes de Aranearum ex ovo 
evolutione, Halle, 1845; of scorpions, H. RatHKe Zur Morphologie, Riga u. Leipzig, 
1837. 4t0. pp. 17—34. Pl. 1. figs. r—11; compare also the shorter description of these 
last observations and of those of HERoLp in Burpacu Die Physiologie als Erfarungs- 
wessenschaft, 2te Ausg. 11, 1837. s. 242—248. 
°? De GuER Mém. pour servir & U Hist. des Ins. vu. pp. 195, 196, Pl. 18, figs. 
1I—J4, 
