Between ova with a segmentation like that of Insects, and those with a segmentation like that of Penæus, there is more than one intermediate form. The Eupagurus type, with the division of the first nucleus into eight nuclei before the division of the ovum, must be regarded in this light; but the most instructive example of such a transitional type of segmentation is that afforded by Spiders[63].
Fig. 53. Three stages in the segmentation of Philodromus Limbatus.
(After Hub. Ludwig.)
The first phenomenon which can be observed after impregnation is the conglomeration of the yolk spheres into cylindrical columns, which finally assume a radiating form diverging from the centre of the egg. In the centre of the radiate figure is a protoplasmic mass, probably containing a nucleus, which sends out protoplasmic filaments through the columns ([fig. 53] A). After a certain period of repose the figure becomes divided into two rosette-like masses, which remain united for some time by a protoplasmic thread: this thread is finally ruptured ([fig. 53] B). The whole egg does not in this process divide into two segments, but merely the radiate figure, which is enclosed in a finely granular material. The two rosettes next become simultaneously divided, giving rise to four rosettes ([fig. 53] C): and the whole process is repeated with the same rhythm as in a regular segmentation till there are formed thirty-two rosettes in all ([fig. 54] A). The rosettes by this time have become simple columns, which by mutual pressure arrange themselves radiately around the centre of the egg, which however they do not quite reach.
Fig. 54. Surface view and optical section of a late stage in the segmentation of Philodromus Limbatus (Koch). (After Hub. Ludwig.)
bl. blastoderm; yk. yolk spheres.
When only two rosettes are present the protoplasm with its nucleus occupies a central position in each rosette, but gradually, in the course of the subsequent subdivisions, it travels towards the periphery, and finally occupies, when the stage with thirty-two rosettes is reached, a peripheral position. The peripheral protoplasm next becomes separated off as a nucleated layer ([fig. 54] B). It forms the proper blastoderm, and in it the nuclei rapidly multiply and finally around each an hexagonal or polygonal area of protoplasm is marked off; and a blastoderm, formed of a single layer of flattened cells, is thus constituted. The columns within the blastoderm now form ([fig. 54] B) more or less distinct masses, which are stated by Ludwig to be without protoplasm.