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 stomodæum and proctodæum.
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 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).
Fig. 203. Transverse sections through the ventral plate of Agelena labyrinthica at three stages.
A. Stage when about three segments are formed. The mesoblastic plate is not divided into two bands.
B. Stage when six segments are present (fig. 200 B). The mesoblast is now divided into two bands.
C. Stage represented in fig. 200 D. The ventral cords have begun to be formed on thickenings of the epiblast, and the limbs are established.
ep. epiblast; me. mesoblast; me.s. mesoblastic somite; vn. ventral nerve cord; yk. yolk.
The ventral cords are at first composed of as many ganglia as there are segments. The foremost pair, belonging to the segment of the cheliceræ, lie immediately behind the stomodæum, and are as independent of each other as the remaining ganglia. Anteriorly they border on the supra-œsophageal ganglia. When the yolk-sack is formed in connection with the ventral flexure of the embryo, the two nerve cords become very widely separated ([fig. 206], vn) in their middle region. At a later period, at the stage represented in [fig. 201] B, they again become approximated in the ventral line, and delicate commissures are formed uniting the ganglia of the two sides, but there is no trace at this or any other period of a median invagination of epiblast between the two cords, such as Hatschek and other observers have attempted to establish for various Arthropoda and Chætopoda. At the stage represented in [fig. 201] A the nerve ganglia are still present in the abdomen, though only about four ganglia can be distinguished. At a later stage these ganglia fuse into two continuous cords, united however by commissures corresponding with the previous ganglia.
The ganglia of the cheliceræ have, by the stage represented in fig. 201 B, completely fused with the supra-œsophageal ganglia and form part of the œsophageal commissure. The œsophageal commissure is however completed ventrally by the ganglia of the pedipalpi.