'88 Morin, J. From the Summary in "Lehrbuch der vergleichenden Entwickungsgeschichte der wirbellosen Thiere. Korshelt and Heider, Heft, ii.

'93 Pocock, R. I. Morphology of Arachnida. Annals and Magazine of Natural History, 1893.

'87 Schimkewitsch, W. Étude sur le développment des Araignées. Archives de Biologie, vi, 1887.

All figures, except fig. 10, are magnified 125 diameters.

EXPLANATION OF THE PLATE.

FIG. 1.-A sagittal section showing the coelomic cavities of the VII, VIII and IX somites.

FIG. 2.-A sagittal section through the same region as the preceding at a little later stage. The coelomic pouch of the x somite is seen,

as well as the inpushings of ectoderm behind somites VII and

VIII.

FIG. 3.-A section cut under conditions similar to those of the two preceding. The stage is a little later than fig. 2, and shows the coelomic cavity of somite XI aud further inpushings of the ectoderm. In all of the preceding sections the somatopluric wall is the thicker.

FIG. 4.-In this figure the coelome of somite VII is much reduced in size. The coelome of VII is beginning to thrust itself into the appendage which is marked off by a shallow groove in front and by a deeper inpushing, which has a forward direction, between somites VIII and IX.

FIG. 5.-In this section the inpushing, seen in fig. 4, has gone much farther, dividing the cœlome of somite VIII into two parts. On the inner wall of the appendage are two inpushings which are the beginning of the lamelle of the lung book.

FIG. 6.—A section of the appendage of somite VIII cut at a different angle from the preceding. Here four lamellæ are seen.

FIG. 7.-A sagittal section from a stage just after the reversion of the embryo.

FIG. 8.-A section of the appendage of the IX somite, showing at the inner end of the inpushing, the beginning of the tracheal tubes. FIG. 9.—This is a section of the tracheal region after the reversion of the embryo.

FIG. 10.-This figure is a representation of the ventral surface of an embryo, which is about the same age as those from which sections seen in figs. 5, 6 and 8 were cut.