CHAPTEE XXI. 



ARACHNIDA. 



I. Scorpiones. 

 The Scorpiones are viviparous. The oval or spherical eggs, which 

 are rich in yolk and are each surrounded by a thin membrane, lie in 

 follicles that arise as outgrowths of the walls of the ovarian tubes. 

 Fertilisation takes place either in the ovarian follicles {Euscorpins and 

 Scorpio, Metschnikoff, Laurie), or when the egg has left the follicle 

 and passed into the ovarian tube (Androctonus, Kowalevsky and 

 Schulgin). In the former case the embryo remains in the follicle 

 during the greater part of its development [Scorpio, Joh. Muller), 

 or leaves it when the formation of the germ-band commences 

 (Euscorpius italicus). Further development then takes place in the 

 ovarian tubes or oviducts, which thus function as uteri. At birth 

 the young resemble the adult in their general organisation. 



1. Cleavage and Formation of the Germ-layer. 

 The cleavage of the egg in Scorpions is discoidal. At the pole of 

 the egg, which is directed from the follicle towards the ovarian tube, 

 in the youngest stage as yet observed, there were found a number of 

 cells which formed a small unilaminar cap on the yolk; this is the 

 germ-disc (Fig. 1). The blastoderm spreads gradually from this 

 point, advancing very slowly over the yolk (Fig. 2 A and B). Long 



B 



