Studies on the Onychophora VII. The early embryonic stages ofPeripatopsis, and some general considerations concerning the morphology and phylogeny of the arthropoda

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Abstract
There appear to be four different membranes round the eggs and embryos of the species of Onychophora at various stages of their development, and not two as previously supposed. The first membrane is left behind in the ovary, two further membranes occur in the stages ofPeripatopsishere described, and an outer ‘shell’ is present in oviparous species. InPeripatopsisan inner cuticular membrane is absorbed at the end of segmentation, and an outer chitinous membrane persists until birth. These membranes influence the shape and volume of the embryos, and their properties and functions are described (pp. 489, 491, 527). The uterine eggs and embryos ofPeripatopsisundergo two sudden increases in size. InP. sedgwicki, P. moseleyi, P. balfouriandP. capensisthe unsegmented egg swells and then remains constant in dimensions until the end of segmentation. InP. sedgwicki, P. moseleyi, P. balfouriandP. capensisthe absorption of the inner membrane is followed by another dilatation which expands the blastodermic vesicle to form the large yolk sac. InP. capensisthis dilatation does not take place and no large yolk sac is formed, but the membrane dilates as in the former species. InP. balfourithe dilatation of the embryo is followed by a sudden contraction which causes the early elimination of the yolk sac. The embryo then resembles the corresponding stage ofP. capensis, in both cases floating within very large membranes until the embryos grow to fill them (pp. 489, 491, 501, 504, 512, 529, 537). The unsegmented egg inPeripatopsisbreaks up into a ‘first blastomere’ and a number of nonnucleated ‘cytoplasmic spheres’, all of which float freely in a watery fluid (pp. 496, 529). Segmentation of the ‘blastomere’ results in a disk of blastomeres lying in a single layer against the egg membrane on one side. These cells give rise to the whole of the embryo and not to the ectoderm alone. The disk enlarges and becomes saddle-shaped (pp. 498, 530). InP. moseleyiandP. sedgwickithe disk of blastomeres spreads all round the space within the membrane until its edges meet to form a continuous blastoderm. No blastopore is formed. The disintegrating remains of the cytoplasmic spheres lie in the internal space (p. 499). InP.capensisamdP. balfourithe edges of the disk of blastomeres curl away from the membrane, and ‘large vacuolated cells’ separate from these edges and pass to the concavity. A clear blastopore is present inP. capensisand a transitory or virtual one inP. balfouri. InP. capensisthe blastopore narrows to a slit and finally closes. The large vacuolated cells within form an endodermal lining to an archenteron, which may at first be partially filled by free vacuolated cells (p. 503). InP. balfourithe hemisphere of small blastomeres completely closes round the vacuolated cells; these form a solid mass, and then degenerate. Definitive endoderm arises later from a blastoporal area, as inP. moseleyiandP. sedgwicki(p. 509). The small embryo ofP. balfourithen dilates while that ofP. capensisdoes not. The gastrula ofP. capensisthus corresponds with the hollow single-layered blastodermic vesicle of the other species. The large vacuolated cells form definitive endoderm inP. capensis, they are formed but degenerate inP. balfouri, and they do not occur inP. moseleyiandP. sedgwicki(pp. 530, 535, 536). Evidence in support of the view thatPeripatopsisis secondarily yolkless is provided by the details here presented concerning the form of the egg, segmentation, germ-layer formation, yolk sac, size changes, etc. (p. 535). A germinal disk is formed upon a small part of the blastodermic vesicle inP. moseleyi, P. sedgwickiandP. balfourijust as it starts to dilate. The position of the disk varies specifically. It consists of a posterior thickening on which is situated a blastoporal area, and an anterior thickening from which is formed the mouth-anus. The further growth of the germinal disk and yolk sac is described for the several species. The blastoporal area gives rise to all the endoderm and mesoderm by immigration (pp. 505, 509, 510, 512, 513, 531, 536). When immigration from the blastoporal area first starts inP. capensisandP. balfouria ‘giant cell’ 30 p in diameter sinks in. This cell is clearly glandular inP. capensis, passing droplets of secretion to its surroundings and possessing abundant mitochondria. In both species the cell disappears very soon. The disappearance inP. balfouricoincides with the sudden shrinkage of the yolk sac, which is suggestive of glandular activity of this cell. No such cell is formed inP. moseleyiorP. sedgwickiwhere the yolk sac remains large. This cell is possibly a specialization associated with the elimination of the yolk sac (pp. 512, 514). The formation of the mouth-anus differs in the several species, but in all it is formed after the endoderm is established. InP. capensisthe mouth-anus probably forms by a reopening of the closed blastopore. InP. balfouri,P. moseleyiandP. sedgwickithe mouth-anus arisesde novoby a fusion of the ectoderm and endoderm to form the lips of this organ. Its size varies greatly in the different species. It may open widely and then divide to form mouth and anus, or it may not open until after this division. The mouth-anus does not give endoderm by invagination from the lips (pp. 513, 515, 518, 530, 531, 532, 545). The formation of the mesoderm and of the mesodermal somites is described more fully than before, and with particular reference to the head end of the body. Details are given of the formation, growth and migrations of the somites, and of the origin of their coelomic cavities. The formation of...

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