Fig. 200. Four stages in the development of Agelena labyrinthica.
A. Stage when the ventral plate is very imperfectly differentiated. pr.c. primitive cumulus.
B. Ovum viewed from the side when the ventral plate has become divided into six segments. ch. segment of cheliceræ imperfectly separated from procephalic lobe; pd. segment of pedipalpi.
C. Ventral plate ideally unrolled after the full number of segments and appendages are established. st. stomodæum between the two præ-oral lobes. Behind the six pairs of permanent appendages are seen four pairs of provisional appendages.
D and E. Two views of an embryo at the same stage. D ideally unrolled, E seen from the side. st. stomodæum; ch. cheliceræ; on their inner side is seen the ganglion belonging to them. pd. pedipalpi; pr.p. provisional appendages.
The segments continue to increase in number by the continuous addition of fresh segments between the one last formed and the caudal lobe. By the stage with nine segments the first rudiments of the limbs make their appearance. The first rudiments to appear are those of the pedipalpi and four ambulatory limbs: the cheliceræ, like the segment to which they belong, lag behind in development. The limbs appear as small protuberances at the borders of their segments. By the stage when they are formed the procephalic region has become bilobed, and the two lobes of which it is composed are separated by a shallow groove.
By a continuous elongation the ventral plate comes to form a nearly complete equatorial ring round the ovum, the procephalic and caudal lobes being only separated by a very narrow space, the undeveloped dorsal region of the embryo. This is shewn in longitudinal section in [fig. 204]. In this condition the embryo may be spoken of as having a dorsal flexure. By the time that this stage is reached ([fig. 200] C) the full number of segments and appendages has become established. There are in all sixteen segments (including the caudal lobe). The first six of these bear the permanent appendages of the adult; the next four are provided with provisional appendages; while the last six are without appendages. The further features of this stage which deserve notice are (1) the appearance of a shallow depression (st)—the rudiment of the stomodæum—between the hinder part of the two procephalic lobes; (2) the appearance of raised areas on the inner side of the six anterior appendage-bearing segments. These are the rudiments of the ventral ganglia. It deserves to be especially noted that the segment of the cheliceræ, like the succeeding segments, is provided with ganglia; and that the ganglia of the cheliceræ are quite distinct from the supra-œsophageal ganglia derived from the procephalic lobes. (3) The pointed form of the caudal lobe. In Pholcus (Claparède, No. [442]) the caudal lobe forms a projecting structure which, like the caudal lobe of the Scorpion, bends forward so as to face the ventral surface of the part of the body immediately in front. In most Spiders such a projecting caudal lobe is not found. While the embryo still retains its dorsal flexure considerable changes are effected in its general constitution. The appendages ([fig. 200] D and E) become imperfectly jointed, and grow inwards so as to approach each other in the middle line. Even in the stage before this, the ventral integument between the rudiments of the ganglia had become very much thinner, and had in this way divided the ventral plate into two halves. At the present stage the two halves of the ventral plate are still further separated, and there is a wide space on the ventral side only covered by a delicate layer of epiblast. This is shewn in surface view ([fig. 200] D) and in section in [fig. 203] C.
The stomodæum (st) is much more conspicuous, and is bounded in front by a prominent upper lip, and by a less marked lip behind. The upper lip becomes less conspicuous in later stages, and is perhaps to be compared with the provisional upper lip of Chelifer. Each procephalic lobe is now marked by a deep semicircular groove.
The next period in the development is characterised by the gradual change in the flexure of the embryo from a dorsal to a ventral one; accompanied by the division of the body into an abdomen and cephalo-thorax, and the gradual assumption of the adult characters.
The change in the flexure of the embryo is caused by the elongation of the dorsal region, which has hitherto been hardly developed. Such an elongation increases the space on the dorsal surface between the procephalic and caudal regions, and therefore necessarily separates the caudal and procephalic lobes; but, since the ventral plate does not become shortened in the process, and the embryo cannot straighten itself in the egg-shell, it necessarily becomes ventrally flexed.
If there were but little food-yolk this flexure would naturally cause the whole embryo to be bent in so as to have the ventral surface concave. But instead of this the flexure is at first confined to the two bands which form the ventral plate. These bands, as shewn in [fig. 201] A, acquire a true ventral flexure, but the yolk forms a projection—a kind of yolk-sack as Barrois (No. [441]) calls it—distending the thin integument between the two ventral bands. This yolk-sack is shewn in surface view in [fig. 201] A and in section in [fig. 206]. At a later period, when the yolk has become largely absorbed, the true nature of the ventral flexure becomes quite obvious, since the abdomen of the young Spider, while still in the egg, is found to be bent over so as to press against the ventral surface of the thorax ([fig. 201] B).