THE EMBRYOLOGY OF A SCORPION. 119 



A distinct groove^ the neural groove {n. g.), runs down the 

 middle line and extends some distance into the head-segment. 

 It is due to a thinning of the epiblast in the middle line 

 (figs. 25 and 26, n. g.). The ventral nervous system is 

 formed by a thickening of the epiblast along each side of 

 this groove. 



The epiblast now spreads as a single layer beyond the 

 hypoblast {ep'.) and extends over nearly half the yolk, carrying 

 the amnion with it. This is diagrammatically shown in fig, 27. 

 In the head-segment (fig. 25) the epiblast is irregularly 

 grooved and thickened. This is the commencement of the 

 formation of the cerebral ganglion. In the thoracic somites 

 (fig. 26) the epiblast is very thick and solid at the corners {ap.) 

 where the appendages are about to appear. It is also some- 

 what solid just at each side of the neural groove (w. th.). 

 This is the commencement of the thickening which will form 

 the ventral nervous system. 



The niesoblast is a thin layer in the head-segment (fig. 24, 

 mes.), but shows the coelomic space (cce.) distinctly. This 

 development of a head coelom does not, of course, as Balfour 

 has pointed out, necessarily indicate that the head-segment is 

 equivalent to a body somite. In the body somites (fig. 

 26) the mesoblast is pretty thick and the coelomic space is 

 almost entirely closed up. The mesoblast does not extend 

 across the middle line or beyond the limits of the ventral 

 plate. 



The hypoblast (figs. 25, 26, hy.) shows no change from the 

 last stage but remains as a single layer, except in the tail- 

 segment, where the hypoblastic mass is distinctly visible. 



As the next stage shows the commencement of a large 

 number of new structures, the ventral nervous system, the 

 appendages, &c., it seems advisable to give a short summary 

 of what has taken place so far. 



First Period. 



(1) The blastoderm commences as a single saucer-shaped 

 layer of cells at one end of the egg (Stage A). 



