TRACHEATA. 447 



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 

 (hypodermis and cuticle), also supplies the elements for the 

 nervous system and organs of sense, and for the respiratory 

 sacks, the stomodaeum and proctodaeum. 



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 epi- 

 blast (fig. 200 C). The rudiments of the ventral nerve-cord are 

 for a long time continuous 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 



