VIII 



AKTHROPODA 



239 



As regards the Scorpion, the embryology of which has been 

 worked at by many observers, the latest of whom is Brauer (1894, 

 1895), we find again serious modifications in the early development 

 as compared with that of the spider. The eggs of the scorpion are not 

 laid, but are retained within the body of the mother until develop- 

 ment has so far advanced that the young, when born, have most of 

 the features of the adult. The nucleus of 

 the ripe egg is situated not in the interior c ;ff r 



of the egg but at its surface, and the 

 daughter nuclei, which result from its 

 division, form at first a single -layered 

 blastoderm extending over only a portion 

 of the surface of the egg. The egg is there- , 



germ 



stom 



mes 



FIG. 189. Two transverse sections through the "ger- 

 minal disc," or developing area of the egg of the 

 Scorpion, Euscorpius carpathicus, in two stages. 

 (After Brauer.) 



A, Stage of the formation of the serosa. B, stage of the 

 formation of the amnion. am, beginning amnion ; ect, 

 ectoderm ; end, endodermal nuclei ; mes, mesoderm ; germ,, 

 ^primitive germ cells, probably corresponding to the " primi- 

 tive cumulus " of the spider's egg. 



FIG. 190. Ventral view of em- 

 bryo of the Scorpion Euscor- 

 pius carpathicus showing 

 segments and appendages. 

 (After Brauer.) 



alA'l, the rudiments of the ab- 

 dominal appendages ; ch, rudiment 

 ofchelicera; ch.g, rudiment of the 

 cheliceran ganglion ; c.gr, cephalic 

 groove ; c.l, caudal lobe ; Z 1 " 4 , the 

 rudiments of the walking legs ; ped, 

 rudiment of pedipalp ; stom, open- 

 ing of stomodaeum ; v.g, ganglia of 

 the ventral nerve cord. 



fore telolecithal and its segmentation is 



meroblastic, but this type of telolecithal 



egg, as we can see by comparing it with the 



egg of the spider, must have been secondarily derived from the 



centrolecithal type. 



At a later period cells are budded off from the blastoderm which 

 wander into the yolk. These cells, according to Brauer, eventually 

 form the endodermal epithelium, at a much later period in the develop- 

 ment. At the same time the edges of the blastoderm give rise to a 

 sheet of cells which grows backwards over it and forms a protective 

 cover for it, called the amnion. A little later, a second outer cover- 

 ing of the same kind is formed which is called the serosa (Fig. 189). 



