240 INVEETEBEATA chap. 



A keel is formed by a thickening of the blastoderm as in the spider ; 

 from this keel a layer of cells grows outward on each side beneath 

 the ectoderm, as in the spider and in Limulus, and forms the 

 mesoderm. Behind this primary thickening there is a second thicken- 

 ing formed, as Kishinouye has also described for the spider. Brauer, 

 however, asserts that in the scorpion this secondary thickening, or 

 primitive cumulus as it is called, gives rise to a group of cells which 

 remains unchanged for a great period of development and then gives 

 rise to the genital cells. There is strong presumption that this will 

 eventually be found to be true in the case of the spider also. 



The general history of the later development of the scorpion is 

 very similar to that of the spider in its main outlines, but the 

 following points are to be noted. The egg is cylindrical and the 

 ventral plate only occupies one side, consequently there is no need 

 for reversion. But since the blastoderm only covers a portion of 

 the surface and is reflected at its edges to form protective membranes, 

 the covering of the dorsal surface of the egg with skin is effected by 

 the lateral growth of the ventral plate, and the pushing of its right 

 and left edges (that is to say, the lines of origin of serosa and amnion) 

 farther and farther up towards the dorsal surface, till they meet on 

 the mid-dorsal line, results in the protective membranes being cut 

 off from the egg. 



Brauer could only distinguish two segments in the brain, each 

 marked by a transverse commissure. The lateral eyes remain as open 

 pits of epithelium, and they have no vitelligenous layer. The so-called 

 coelomic cavity of the head or cephalic lobe is, according to him, 

 merely an extension of the coelomic sac belonging to the segment of 

 the chelicera, as may also be the case with both spider and Limulus, 

 in spite of Kishinouye's statement; or perhaps Brauer is mistaken 

 and has confused subsequent fusion with common origin. 



The Malpighian tubes are outgrowths of the posterior end of the 

 mid-gut. Eudiments of excretory organs or so-called " nephridia " 

 are formed as outgrowths from the coelomic sacs in all the segments 

 of the prosoma from the second to the sixth, but only that one in 

 the fifth segment comes to full development and it forms the coxal 

 gland. The coelomic sacs of the abdomen press on the cells which 

 form the genital rudiment, and eventually these cells pass into the 

 interior of the first coelomic sac of this region of the body. The 

 genital ducts arise in exactly the same way as the " nephridia," with 

 which they are no doubt serially homologous. 



The abdominal appendages form at first freely projecting plates, 

 but the first abdominal segment has only vestiges of appendages 

 which soon disappear and the whole segment then becomes in- 

 distinguishable ; the appendages of the second segment form the 

 genital operculum, those of the third pair form the "combs" or 

 pectines. The skin behind the fourth, fifth, sixth, and seventh pairs 

 of appendages becomes tucked in so as to form the lung sacs, and 

 folds on the anterior surfaces of these sacs form the lung books. 



