432 



SCORPIONID^E. 



formed a germinal disc which undergoes a partial segmentation 

 (fib- *93 bl}. A somewhat saucer-shaped one-layered blasto- 

 derm is then formed, which soon becomes thickened in the 

 centre and then divided into two layers. The outer of these 

 is the epiblast, the inner the mesoblast. Beneath the mesoblast 

 there subsequently appear granular cells, which form the 

 commencement of the hypoblast 1 . 



During the formation of the blastoderm a cellular envelope is formed 

 round the embryo. Its origin is doubtful, though it is regarded by 

 Metschnikoff as probably derived from the blastoderm and homologous 

 with the amnion of Insects. It becomes double in the later stages (fig. 195). 



During the differentiation of the three embryonic layers the 

 germinal disc becomes somewhat pyriform, the pointed end 

 being the posterior. At this extremity there is a special thick- 

 ening which is perhaps 

 equivalent to the prim- 

 itive cumulus of Spiders. 

 The germinal disc con- 

 tinues gradually to spread 

 over the yolk, but the 

 original pyriform area is 

 thicker than the remain- 

 der, and is marked off 

 anteriorly and posterior- 

 ly by a shallow furrow. 

 It constitutes a structure 

 corresponding with the 

 ventral plate of other 

 Tracheata. It soon be- 

 comes grooved by a 

 shallow longitudinal fur- 



row (fig. 194 A) which 

 subsequently becomes 

 less distinct. It is then 

 divided by two transverse lines into three parts 2 . 



FIG. 194. THREE SURFACE VIEWS OF THE 

 VENTRAL PLATE OF A DEVELOPING SCORPION. 

 (After Metschnikoff.) 



A. Before segmentation. 



B. After five segments have become formed. 



C. After the appendages have begun to be 



formed. 



1 The origin of the hypoblast cells, if such these cells are, is obscure. Metschnikoff 

 doubtfully derives them from the blastoderm cells ; from my investigations on Spiders 

 it appears to me more probable that they originate in the yolk. 



' The exact fate of the three original segments is left somewhat obscure by 



