372 



ARACHNID A. 



ganglia from the epiblast, the cells lining the grooves become in- 

 voluted and detached from the skin, and form an integral part of 

 the supra-cesophageal ganglia. 



At the stage represented in fig. 201 B the siipra-cesophageal ganglia 

 are completely detached from the epiblast, and ai-e constituted of the 

 following parts: (1) A dorsal section formed of two hemispherical lobes, 

 mainly formed of the invaginated lining of the semicircular grooves. The 

 original lurnen of the groove is still present on the outer side of these 

 lobes. (2) Two central masses, one for each ganglion, formed of pnncti- 

 forin tissue, and connected by a transverse commissure. (3) A ventral 

 anterior lobe. (4) The original ganglia of the chelicene, which form the 

 ventral parts of the ganglia '. 



The later stages in the development of the nervous system have not 

 been worked out. 



The development of the nervous system in the Scorpion is almost 

 identical with that in Spiders, but Metschnikoff believes, though without 

 adducing satisfactory evidence, that the median integument between the 

 two nerve cords assists in forming the ventral nerve cord. Grooves are 

 present in the supra-cesophageal ganglia similar to those in Spiders. 



The mesoblast. The history of the mesoblast, up to the formation 

 of a ventral plate subjacent to the thickened plate of epiblast, has been 

 already given. The ventral plate is shewn in fig. 203 A. It is seen 

 to be formed mainly of small cells, but some large cells are shewn in 



10 



FIG. 204. LONGITUDINAL SECTION THROUGH AN EMBRYO OF AGELENA LABYRINTHICA. 



The section is through an embryo of the same age as that represented in fig. 200 C, 

 and is taken slightly to one side of the middle line so as to shew the relation of the 

 mesoblastic somites to the limbs. In the interior are seen the yolk segments and 

 their nuclei. 



1 16. the segments; pr.L procephalic lobe ; do. dorsal integument. 



the act of passing into it from the yolk. During a considerable section 



1 For further details vide self, No. 440. 



