The ventral ganglia, though not the supra-œsophageal, have become separated from the epiblast.
The yolk nuclei, each surrounded by protoplasm as before, are much more numerous.
In other respects there are no great changes in the internal features.
In my next stage, represented in Pl. 30, figs. 8a, and 8b, a very considerable advance has become effected. In the first place the dorsal surface has increased in length to rather more than one half the circumference of the ovum. The dorsal region has, however, not only increased in length, but also in definiteness, and a series of transverse markings (figs. 8a and b), which are very conspicuous in the case of the four anterior abdominal segments (the segments with rudimentary appendages), have appeared, indicating the limits of segments dorsally. The terga of the somites may, in fact, be said to have become formed. The posterior terga (fig. 8a) are very narrow compared to the anterior.
The caudal protuberance is more prominent than it was, and somewhat bilobed; it is continued on each side into one of the bands, into which the ventral plate is divided. These bands, as is best seen in side view (fig. 8b), have a ventral curvature, or, perhaps more correctly, are formed of two parts, which meet at a large angle open towards the ventral surface. The posterior of these parts bears the four still very conspicuous provisional appendages, and the anterior the six pairs of thoracic appendages. The four ambulatory appendages are now seven-jointed, as in the adult, but though longer than in the previous stage they do not any longer cross or even meet in the middle line, but are, on the contrary, separated by a very considerable interval. This is due to the great distension by the yolk of the ventral part of the body, in the interval between the two parts of the original ventral plate. The amount of this yolk may be gathered from the section (Pl. 32, fig. 18). The pedipalpi carry a blade on their basal joint. The cheliceræ no longer appear to spring from an independent postoral segment.
There is a conspicuous lower lip, but the upper is less prominent than before. Sections at this stage shew that the internal changes have been nearly as considerable as the external.
The dorsal region is now formed of a (1) flattened layer of epiblast cells, and a (2) fairly thick layer of large and rather characteristic cells which any one who has studied sections of spider's embryos will recognize as derivatives of the yolk. These cells are not, therefore, derived from prolongations of the somatic and splanchnic layers of the already formed somites, but are new formations derived from the yolk. They commenced to be formed at a much earlier period, and some of them are shewn in the longitudinal section (Pl. 31, fig. 15). In the next stage these cells become differentiated into the somatic and splanchnic mesoblast layers of the dorsal region of the embryo.
In the dorsal region of the abdomen the heart has already become established. So far as I have been able to make out it is formed from a solid cord of the cells of the dorsal region. The peripheral layer of this cord gives rise to the walls of the heart, while the central cells become converted into the corpuscles of the blood.
The rudiment of the heart is in contact with the epiblast above, and there is no greater evidence of its being derived from the splanchnic than from the somatic mesoblast; it is, in fact, formed before the dorsal mesoblast has become differentiated into two layers.
In the abdomen three or four transverse septa, derived from the splanchnic mesoblast, grow a short way into the yolk. They become more conspicuous during the succeeding stage, and are spoken of in detail in the description of that stage. In the anterior part of the thorax a longitudinal and vertical septum is formed, which grows downwards from the median dorsal line, and divides the yolk in this region into two parts. In this septum there is formed at a later stage a vertical muscle attached to the suctorial part of the stomodæum.