Fig. 205. Section through the procephalic lobes of an embryo of Agelena labyrinthica.
The section is taken from an embryo of the same age as fig. 200 D.
st. stomodæum; gr. section through semicircular groove in procephalic lobe; ce.s. cephalic section of body cavity.
The first important change takes place when about six somites are established; the mesoblast then becomes divided into two lateral bands, shewn in section in [fig. 203] B, which meet however in front in the procephalic lobes, and behind in the caudal lobes. Very shortly afterwards these bands become broken up into a number of parts corresponding to the segments, each of which soon becomes divided into two layers, which enclose a cavity between them (vide [fig. 204] and [fig. 207]). The outer layer (somatic) is thicker and attached to the epiblast, and the inner layer (splanchnic) is thinner and mainly, if not entirely, derived (in Agelena) from cells which originate in the yolk. These structures constitute the mesoblastic somites. In the appendage-bearing segments the somatic layer of each of them, together with a prolongation of the cavity, is continued into the appendage ([fig. 203] C). Since the cavity of the mesoblastic somites is part of the body cavity, all the appendages contain prolongations of the body cavity. Not only is a pair of mesoblastic somites formed for each segment of the body, but also for the procephalic lobes ([fig. 205]). The mesoblastic somites for these lobes are established somewhat later than for the true segments, but only differ from them in the fact that the somites of the two sides are united by a median bridge of undivided mesoblast. The development of a somite for the procephalic lobes is similar to what has been described by Kleinenberg for Lumbricus (p. [339]), but must not be necessarily supposed to indicate that the procephalic lobes form a segment equivalent to the segments of the trunk. They are rather equivalent to the præ-oral lobe of Chætopod larvæ. When the dorsal surface of the embryo is established a thick layer of mesoblast becomes formed below the epiblast. This layer is not however derived from an upgrowth of the mesoblast of the somites, but from cells which originate in the yolk. The first traces of the layer are seen in [fig. 204], do, and it is fully established as a layer of large round cells in the stage shewn in [fig. 206]. This layer of cells is seen to be quite independent of the mesoblastic somites (me.s). The mesoblast of the dorsal surface becomes at the stage represented in [fig. 201] B divided into splanchnic and somatic layers, and in the abdomen at any rate into somites continuous with those of the ventral part of the mesoblast. At the lines of junction of successive somites the splanchnic layer of mesoblast dips into the yolk, and forms a number of transverse septa, which do not reach the middle of the yolk, but leave a central part free, in which the mesenteron is subsequently formed. At the insertion of these septa there are developed widish spaces between the layers of somatic and splanchnic mesoblast, which form transversely directed channels passing from the heart outwards. They are probably venous. At a later stage the septa send out lateral offshoots, and divide the peripheral part of the abdominal cavity into a number of compartments filled with yolk. It is probable that the hepatic diverticula are eventually formed in these compartments.
Fig. 206. Transverse section through the thoracic region of an embryo of Agelena labyrinthica.
The section is taken from an embryo of the same age as fig. 201 A, and passes through the maximum protuberance of the ventral yolk-sack.