The Development and Affinities of the Pauropoda, based on a Study of Pauropus silvaticus

ABSTRACT 1. The minute egg is heavily yolked, and is devoid of periplasm or vitelline membrane. Shortly before laying, the germinal vesicle phase gives way to one in which the chromosomes reappear as thirteen ‘tetrads’ in a central clump of cytoplasm. The latter moves to the periphery, and in this first meiotic prophase the egg is laid. The ‘polar bodies’ do not separate from the egg. In most eggs they degenerate very rapidly. Male and female pro-nuclei fuse in the centre of the egg to form a resting nucleus. 2. Cleavage is total and unsynchronized, the cleavage-cells becoming arranged as yolk-pyramids around a central blastocoele. This blastula persists up to about the eighty-cell stage, when a gastrula is formed by migration of one, or at most two cells, from the layer of yolk-pyramids into the blasto-coele to form the endoderm. 3. The gastrula is succeeded by a blastoderm of the familiar myriapodinsect type ; the cleavage nuclei, except those of the endoderm, move into the peripheral layer of accumulating cytoplasm, while the internal cell-partitions break down. Total cleavage is thereby replaced by a superficial cleavage within the blastoderm. A blastodermic cuticle is secreted. 4. Out of the blastoderm there now differentiates the germ-band; it is of the usual elongate myriapod type, and extends over the anterior and posterior poles of the egg on to its upper half. At no time does a ventral flexure form. The rest of the blastoderm becomes much thinned out and is a provisional body-wall. 5. From the inner surface of the blastoderm isolated cells with enlarging nuclei have meantime migrated as yolk-cells into the yolk; from them will eventually form the fat-body of the larva. 6. During the development of the germ-band the latter becomes the source of the developing mesoderm, cells separating from it in great numbers along its length, and so coming to form a second, and at first very irregular layer, internal to the ectoderm. 7. The early developmental processes are of the usual myriapod type, except for the presence of an easily recognized gastrula. The survival of the latter is of importance for the interpretation of the specialized myriapod insect type of ontogeny; in particular, the blastoderm phase is found to be a post-gastrula stage, and not a blastula as commonly believed. 8. Stomodaeum and proctodaeum are the first structures to appear in the germ-band. The formation of the head-lobes soon follows. Then the Anlagen of the antennae arise, being at first post-oral in position ; and after them appear the Anlagen of the mandibles and maxillae. The premandibular segment does not bear even the rudiments of appendages. The segment behind the maxillary segment is the collum segment, without appendage-Anlagen, and it remains part of the abdomen; there is, therefore, no second maxillary segment. The Anlagen of the first, second, and third legs then appear in succession. Intersegmental lines form only in the advanced embryo ; when these eventually appear, they reveal two segments behind the fourth abdominal (third leg-bearing), namely the fifth abdominal and anal segments. The fifth segment of the embryo becomes the fifth of the adult animal, so that the teloblastic formation of new segments must proceed by the budding off of new segments from the stationary anal segment. 9. The embryo now slowly swells, presumably owing to absorption of water, and so outgrows the available space within the egg. A gradually enlarging rent appears in the egg-shell, and after several days the embryo emerges from it in a quiescent ‘pupoid’ phase. Unlike the ‘pupa’ of other myriapods, it shows but little resemblance to the future larva. Within the pupa the first teloblastic segment (sixth abdominal) appears. 10. The development of the abdominal segments, both in the embryo and in the pupa and larva, shows that there are no ‘diplosegments’ in Pauropus-, the tergites are derivatives of simple segments, and the apparent ‘diplopody’ arises from the presence of a wedge-shaped segment, with reduced tergal wall, behind the tergite-bearing segments. 11. The differentiation of the head-capsule out of the primitive headsegments is attended by the familiar inturning of the sternal wall of the postoral segments to form the floor of the pre-oral cavity; and by the curving forward of the more lateral parts of the segments towards the front of the head. The antennae are thereby carried into a completely pre-oral position. The pre-mandibular epidermis becomes rolled under to form the roof of the pre-oral cavity (inferior surface of clypeus). 12. The head is composed of a procephalon, in which pre-antennary, antennary, and pre-mandibular segments are represented; and of a gnatho-cephalon, consisting of only two segments, the mandibular and maxillary. Superlinguae, derived from the mandibular epidermis, are present. The mandibles are closed in by the in turned margins of the clypeus; they are unsegmented and adapted to a diet of semi-fluid food. The maxillae display a cardo, a stipes, and a lacinia. Between them is the intermaxillary plate (sternite of maxillary segment). 13. At no time does the germ-band display a ventral flexure. Embryonic membranes are absent. An embryonic ‘dorsal organ’ appears, but it is not of the type found in Symphyla, Collembola, and Campodea. 14. In the early germ-band the mesoderm tends to aggregate laterally into two bands of cells, from which the succession of somites arises. Although many of the somites soon display very small coelomic cavities, they remain poorly developed, and coelomoducts do not appear. Since there are no blood-vessels in Pauropus, they do not contain any vasoblasts ; they are, more over, quite unique in that they do not even contribute any mesoderm to the mid-gut wall, the splanchnic mesoderm arising entirely from the mesoderm of the stomodaeum. Nor do they participate in the formation of the genital rudiment. The dorso-lateral muscles also are not derived from the somites. 15. Between the rows of somites is a layer of unsegmented ‘median mesoderm’ ; out of it develops the genital tube, as well as a median band of neuroglia ( ?) tissue in the nerve-cord. 16. Although the somites are diminutive, a complete set is present. They are the pre-antennary (vestigial), antennary, pre-mandibular, mandibular, maxillary, collum, second, third, and fourth abdominal somites; a fifth abdominal and very small anal somite form in the more advanced embryo. In addition to these there is also a small clump of ‘teloblastic mesoderm’ arising in the late embryo from the mesoderm that is heaped up in front of the proctodaeum ; from it is generated the mesoderm of the larva. 17. From the pre-antennary somite arise the buccal dilator muscles; from the antennary and mandibular somites arise the muscles of the antennae and mandibles respectively. 18. From the pre-mandibular somite arises the large pre-mandibular gland; it opens to the side of the mandibles and is evidently a salivary gland. Its duct seems to be of ectodermal origin. There is no associated ‘end-sac’. In Symphyla a pre-mandibular gland is present up to the time the larva leaves the egg, when it degenerates, leaving only its nephrocytes ; in Pauropus alone among myriapods is it known to survive, though vestiges of it are found both in chilopods (‘lymphoid tissue’) and insects (sub-oesophageal bodies). ‘ 19. From the maxillary somite there develops, in addition to the muscles of the maxilla, the maxillary gland. The latter has an ‘end-sac’, which displays nephrocytic action to trypan blue injected into the blood. 20. The somites of the collum and other abdominal segments, as well as the anal segment, give origin to nothing but myoblasts from which most of the muscles of the respective segments develop. 21. In addition to the glands already referred to, there are present: (a) clypeal glands, that arise from the epidermis of the clypeus ; (6) pseudocular glands, lying adjacent to the pseudoculi, and derived from the epithelium of the latter; (c) large intermaxillary glands, derived from the maxillary sternum. 22. The mid-gut epithelium is formed from the endoderm of the gastrula, the cells gradually losing their yolk and slowly increasing in number. The mesoderm of the mid-gut is derived from the mesodçrm that is heaped up along the stomodaeum, whence it spreads back as an arching roof to the endoderm, the immediately underlying cells of which become arranged into an epithelium, within which excretory concretions, similar to those of the adult mid-gut, soon appear. The floor of the mid-gut remains for long free from any mesoderm; from it develops a ventral band of enlarged mid-gut cells, permanently free, from excretory concretions. The hindermost tip of the mid-gut is of proctodaeal origin. The lumen of the mid-gut does not communicate with those of the stomodaeum and proctodaeum till shortly before the larva emerges. 23. The two Malpighian tubes arise from the anterior tip of the proctodaeum; they do not seem to be fünctional excretory tubes, since in the growing larva they begin to display a markedly degenerate character. 24. The genital rudiment does not arise out of the somites, but from t