68o NOTES ON THE DEVELOPMENT OF THE ARANEINA. 



is interesting, as this number is permanent in Insects and in 

 Peripatus. 



The next stage figured forms the last of the third period 

 (PI. 30, figs. 7 and yd). The ventral plate is still rolled round the 

 egg (fig. 7), and the end of the tail and the procephalic lobes 

 nearly meet dorsally, so that there is but a very slight develop- 

 ment of the dorsal region. There are the same number of 

 segments as before, and the chief differences in appearance be- 

 tween the present and the previous stage depend upon the fact 

 (i) that the median ventral integument between the nerve 

 ganglia has become wider, and at the same time thinner ; (2) 

 that the limbs have become much more developed; (3) that 

 the stomodaeum is definitely established; (4) that the pro- 

 cephalic lobes have undergone considerable development. 



Of these features, the three last require a fuller description. 

 The limbs of the two sides are directed towards each other, and 

 nearly, meet in the ventral line. The chelicerae are two-jointed, 

 and terminate in what appear like rudimentary chelae, a fact 

 which perhaps indicates that the spiders are descended from 

 ancestors with chelate chelicerae. The four embryonic post- 

 ambulatory appendages are now at the height of their develop- 

 ment. 



The stomodaeum (PI. 30, fig. 7, and PI. 31, fig. 17, sf) is a 

 deepish pit between the two procephalic lobes, and distinctly in 

 front of the segment of the cheliceran. It is bordered in front by 

 a large, well-marked, bilobed upper lip, and behind by a smaller 

 lower lip. The large upper lip is a temporary structure, to be 

 compared, perhaps, with the gigantic upper lip of the embryo of 

 Chelifer (cf. Metschnikoff). On each side of and behind the 

 mouth two whitish masses are visible, which are the epiblastic 

 thickenings which constitute the ganglia of the chelicerae (PI. 30, 

 fig- 7> ch. g). 



The procephalic lobes (pr. /) now form two distinct masses, 

 and each of them is marked by a semicircular groove, dividing 

 them into a narrower anterior and a broader posterior division. 



In the region of the trunk the general arrangement of the 

 germinal layers has not altered to any great extent. The ven- 

 tral ganglionic thickenings are now developed in all the segments 

 in the abdominal as well as in the thoracic region. The individ- 



