Araneina. The eggs of true Spiders are either deposited in nests made specially for them, or are carried about by the females. Species belonging to a considerable number of genera, viz. Pholcus, Epeira, Lycosa, Clubione, Tegenaria and Agelena have been studied by Claparède (No. [442]), Balbiani (No. [439]), Barrois (No. [441]) and myself (No. [440]), and the close similarity between their embryos leaves but little doubt that there are no great variations in development within the group.
Fig. 198. Three stages in the development of chelifer.
(After Metschnikoff.)
pd. pedipalpi; ab. abdomen; an.i. anal invagination; ch. cheliceræ.
The ovum is enclosed in a delicate vitelline membrane, enveloped in its turn by a chorion secreted by the walls of the oviduct. The chorion is covered by numerous rounded prominences, and occasionally exhibits a pattern corresponding with the areas of the cells which formed it. The segmentation has already been fully described, pp. [118] and [119]. At its close there is present an enveloping blastoderm formed of a single layer of large flattened cells. The yolk within is formed of a number of large polygonal segments; each of which is composed of large yolk-spherules, and contains a nucleus surrounded by a layer of protoplasm, which is prolonged into stellate processes holding together the yolk-spherules. The nucleus, surrounded by the major part of the protoplasm of each yolk cell, appears, as a rule, to be situated not at the centre, but on one side of its yolk segment.
The further description of the development of Spiders applies more especially to Agelena labyrinthica, the species which formed the subject of my own investigations.
The first differentiation of the blastoderm consists in the cells of nearly the whole of one hemisphere becoming somewhat more columnar than those of the other hemisphere, and in the cells of a small area near one end of the thickened hemisphere becoming distinctly more columnar than elsewhere, and two layers thick. This area forms a protuberance on the surface of the ovum, originally discovered by Claparède, and called by him the primitive cumulus. In the next stage the cells of the thickened hemisphere of the blastoderm become still more columnar; and a second area, at first connected by a whitish streak with the cumulus, makes its appearance. In the second area the blastoderm is also more than one cell deep ([fig. 199]). It will be noticed that the blastoderm, though more than one cell thick over a large part of the ventral surface, is not divided into distinct layers. The second area appears as a white patch and soon becomes more distinct, while the streak continued to it from the cumulus is no longer visible. It is shewn in surface view in [fig. 200] A. Though my observations on this stage are not quite satisfactory, yet it appears to me probable that there is a longitudinal thickened ridge of the blastoderm extending from the primitive cumulus to the large white area. The section represented in [fig. 199], which I believe to be oblique, passes through this ridge at its most projecting part.
