370 ARACHNIDA. 



of the centrolecitlial type, while that of Scorpio, though apparently 

 meroblastic, is probably to be regarded in the same light (vide p. 91> 

 and p. 359). The early development begins in the Scorpion and 

 Spiders with the formation of a ventral plate, and there can be but 

 little doubt that Chelifer is provided with an homologous structure, 

 though very probably modified, owing to the small amount of food- 

 yolk and early period of hatching. 



The history of the layers and their conversion into the organs has 

 been studied in the case of the Scorpion (Metschnikoff, No. 434), 

 and of the Spiders ; and a close agreement has been found to obtain 

 between them. 



It will be convenient to take the latter group as type, and simply 

 to call attention to any points in which the two groups differ. 



The epiblast. The epiblast, besides giving rise to the skin (hypo- 

 dermis and cuticle), also supplies the elements for the nervous system 

 and organs of sense, and for the respiratory sacks, the stomodseum and 

 proctodfeum. 



At the period when the mesoblast is definitely established, the 

 epiblast is formed of a single layer of columnar cells in the region of 

 the ventral plate, and of a layer of flat cells over other parts of the 

 yolk. 



When about six segments are present the first changes take place. 

 The epiblast of the ventral plate then becomes somewhat thinner in 

 the median line than at the two sides (fig. 203 B). In succeeding 

 stages the contrast between the median and the lateral parts becomes 

 still more marked, so that the epiblast becomes finally constituted of 

 two lateral thickened bands, which meet in front in the procephalic 

 lobes, and behind in the caudal lobe, and are elsewhere connected by 

 a very thin layer (fig. 203 C). Shortly after the appendages begin 

 to be formed, the first rudiments of the ventral nerve-cord become 

 established as epiblastic thickenings on the inner side of each of the 

 lateral bands. The thickenings of the epiblast of the two sides are 

 quite independent, as may be seen in fig. 203 C, vn, taken from a 

 stage somewhat subsequent to their first appearance. They are 

 developed from before backwards, but either from the first, or in any 

 case very soon afterwards, cease to form uniform thickenings, but 

 constitute a linear series of swellings the future ganglia connected 

 by very short less prominent thickenings of the epiblast (fig. 200 C). 

 The rudiments of the ventral nerve-cord are for a long time con- 

 tinuous with the epiblast, but shortly after the establishment of the 

 dorsal surface of the embryo they become separated from the epiblast 

 and constitute two independent cords, the histological structure of 

 which is the same as in other Tracheata (fig. 206, vn). 



The ventral cords are at first composed of as many ganglia as 

 there are segments. The foremost pair, belonging to the segment of 

 the cheliceraB, lie immediately behind the stomodseum, and are as 

 independent of each other as the remaining ganglia. Anteriorly 

 they border on the supra-oesophageal ganglia. When the yolk sack is 



