General Conclusions.

Without attempting to compare at length the development of the spiders with that of other Arthropoda, I propose to point out a few features in the development of spiders, which appear to show that the Arachnida are undoubtedly more closely related to the other Tracheata than to the Crustacea.

The whole history of the formation of the mesoblast is very similar to that in insects. The mesoblast in both groups is formed by a thickening of the median line of the ventral plate (germinal streak).

In insects there is usually formed a median groove, the walls of which become converted into a plate of mesoblast. In spiders there is no such groove, but a median keel-like thickening of the ventral plate (Pl. XX, fig. 11), is very probably an homologous structure. The unpaired plate of mesoblast formed in both insects and Arachnida is exactly similar, and beomes divided, in both groups, into two bands, one on each side of the middle line. Such differences as there are between Insects and Arachnida sink into insignificance compared with the immense differences in the origin of the mesoblast between either group, and that in the Isopoda, or, still more, the Malacostraca and most Crustacea. In most Crustacea we find that the mesoblast is budded off from the walls of an invagination, which gives rise to the

mesenteron.

In both spiders and Myriopoda, and probably insects, the mesoblast is subsequently divided into somites, the lumen of which is continued into the limbs. In Crustacea mesoblastic somites have not usually been found, though they appear occasionally to occur, e. g. Mysis, but they are in no case similar to those in the Tracheata.

In the formation of the alimentary tract, again, the differences between the Crustacea and Tracheata are equally marked, and the Arachnida agree with the Tracheata. There is generally in Crustacea an invagination, which gives rise to the mesenteron. In Tracheata this never occurs. The proctodæum is usually formed in Crustacea before or, at any rate, not later than the stomodæum.1 The reverse is true for the Tracheata. In Crustacea the proctodæum and stomodæum, especially the former, are very long, and usually give rise to the greater part of the alimentary tract, while the mesenteron is usually short.

If Grobben's account of the development of Moina is correct this statement must be considered not to be universally true.

In the Tracheata the mesenteron is always considerable, and the proctodæum is always short. The derivation of the Malpighian bodies from the proctodæum is common to most Tracheata. Such organs are not found in the Crustacea.

With reference to other points in my investigations, the evidence which I have got that the cheliceræ are true postoral appendages supplied in the embryo from a distinct postoral ganglion, confirms the conclusions of most previous investigators, and shows that these appendages are equivalent to the mandibles, or possibly the first pair of maxilla of other Tracheata. The invagination, which I have found, of part of a groove of epiblast in the formation of the supraoesophageal ganglia is of interest, owing to the wide extension of a similar occurrence amongst the Tracheata.

The wide divarication of the ventral nerve cords in the embryo renders it easy to prove that there is no median invagination of epiblast between them, and supports Kleinenberg's observations on Lumbricus as to the absence of this invagination. I have further satisfied myself as to the absence of such an invagination in Peripatus. It is probable that Hatschek and other observers who have followed him are mistaken in affirming the existence of such an invagination in either the Chatopoda or the Arthropoda.

The observations recorded in this paper on the yolk cells and their derivations are, on the whole, in close harmony with the observations of Dohrn, Bobretsky, and Graber, on Insects. They show, however, that the first formed mesoblastic plate does not give rise to the whole of the mesoblast, but that during the whole of embryonic life the mesoblast continues to receive accessions of cells derived from the cells of the yolk.

Araneina.

6

1. Balbiani, "Mémoire sur le Développement des Araneides," Ann. Sci. Nat.,' series v, vol. xvii, 1873.

2. J. Barrois, "Recherches s. 1. Développement des Araignées," 'Journal de 1. Anat. e. de la Physiol.,' 1878.

3. E. Claparède, 'Recherches s. l'Evolution des Araignées,' Utrecht, 1860.

4. Herold, 'De Génératione Araniorum in Ovo,' Marburg, 1824. 5. H. Ludwig, "Ueb. d. Bildung des Blastoderm," bie d. Spinnea, 'Zeit. f. wiss. Zool.,' vol. xxvi, 1876.

EXPLANATION OF PLATES XIX, XX, AND XXI,

Illustrating Mr. F. M. Balfour's Notes on the Development of the Araneina.

PLATE XIX.

Complete List of Reference Letters.

ch. g. Ganglion of cheliceræ. c. I. Caudal lobe. ch. Cheliceræ. pd. Pedipalpi. pr. l. Præoral lobe. pp. pp. etc. Provisional appendages. p. c. Primitive cumulus. sp. Spinnerets. st. Stomodæum.

I-IV. Ambulatory appendages. 1-16. Postoral segments. FIG. 1.—Ovum, with primitive cumulus and streak proceeding from it. FIG. 2.-Somewhat later stage, in which the primitive cumulus is still visible. Near the opposite end of the blastoderm is a white area, which is probably the rudiment of the procephalic lobe.

FIG. 3a and 36.-View of an embryo from the ventral surface and from the side when six segments have become established.

FIG. 4.-View of an embryo, ideally unrolled, when the first rudiments of the appendages become visible.

FIG. 5.-Embryo ideally unrolled at the stage when all the appendages have become established.

FIG. 6.-Somewhat older stage, when the limbs begin to be jointed. Viewed from the side.

FIG. 7.-Later stage, viewed from the side.

FIG. 7a.-Same embryo as fig. 7, ideally unrolled.

FIG. 8a and 86.-View from the ventral surface and from the side of

an embryo, after the ventral flexure has considerably advanced. FIG. 9.-Somewhat older embryo, viewed from the ventral surface.

PLATES XX AND XXI.

Complete List of Reference Letters.

ao. Aorta. ab. g. Abdominal nerve cord. ch. Cheliceræ. ch. g. Ganglion of cheliceræ. ep. Epiblast. ht. Heart. hs. Hemispherical lobe of supra-oesophageal ganglion. 7. 1. Lower lip. m. Muscles. me. Mesoblast. mes. Mesenteron. mp.g. Malpighian tube. ms. Mesoblastic somite. a. Esophagus. p. c. Pericardium. pr. Proctodæum (rectum). pd. Pedipalpi. pd. g. Ganglion of pedipalpi. pr. c. Primitive cumulus. s. Septum in abdomen. so. Somatopleure. sp. Splanchnopleure. st. Stomodæum, su. Suctorial apparatus. su. g. Supra-cesophageal ganglion. th. g. Thoracic ganglion. v. g. Ventral nerve cord. yk. Yolk. y. c. Cells derived from yolk. y. n. Nuclei of yolk cells.

Ig-IV g. Ganglia of ambulatory limbs.

1-16. Postoral segments.

FIG. 10.-Section through an ovum, slightly younger than fig. 1. Showing the primitive cumulus and the columnar character of the cells of one half of the blastoderm.

FIG. 11.-Section through an embryo of the same age as fig. 2. Showing the median thickening of the blastoderm.

FIG. 12.-Transverse section through the ventral plate of a somewhat older embryo. Showing the division of the ventral plate into epiblast and mesoblast.

FIG. 13.-Section through the ventral plate of an embryo of the same age as fig. 3, showing the division of the mesoblast of the ventral plate into two mesoblastic bands.

FIG. 14.-Transverse section through an embryo of the same age as fig. 5, passing through an abdominal segment above and a thoracic segment below.

FIG. 15-Longitudinal section slightly to one side of the middle line through an embryo of the same age.

FIG. 16.-Tranverse section through the ventral plate in the thoracic region of an embryo of the same age as fig. 7.

FIG. 17.-Transverse section through the procephalic lobes of an embryo of the same age. gr. Section of hemicircular groove in procephalic lobe.

FIG. 18.-Transverse section through the thoracic region of an embryo of the same age as fig. 8.

FIG. 19.-Section through the procephalic lobes of an embryo of the

same age.

FIG. 20 a, b, c, d, e.-Five sections through an embryo of the same age as fig. 9. a and b are sections through the procephalic lobes, c through the front part of the thorax. d cuts transversely the posterior parts of the thorax, and longitudinally and horizontally the ventral surface of the abdomen. e cuts the posterior part of the abdomen longitudinally and horizontally, and shows the commencement of the mesenteron.

FIG. 21.-Longitudinal and vertical section of an embryo of the same age. The section passes somewhat to one side of the middle line, and shows the structure of the nervous system.

FIG. 22.-Transverse section through the dorsal part of the abdomen of an embryo of the same stage as fig. 9.