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The Development of the Cape Species of Peripatus

1886; The Company of Biologists; Volume: S2-26; Issue: 102 Linguagem: Inglês

10.1242/jcs.s2-26.102.175

1477-9137

Adam C. Sedgwick,

Marine and coastal plant biology

The Segmentation is apparently complete, the ovum appearing to divide into ectoderm and endoderm cells.The so-called endoderm cells are at first without a distinct nucleus, they do not get a nucleus until just before the gastrula stage.All the cells of the ovum, ectodermal as well as endodermal, are connected together by a fine protoplasmic reticulum, which is placed, as are also the cells, immediately beneath the egg membrane, and therefore around a central space.Each ectoderm cell consists of a central nucleus around which is a close protoplasmic spongework, which, at the outer parts of the so-called cell, becomes of a gradually looser nature until it runs into the spongework of the surrounding cells.Each endoderm mass consists of a central denser spongework which gradually becomes looser towards the periphery of the mass until it is continued into a fine reticulum. The endoderm masses are far apart from each other and are connected by this reticulum.The continuity of the various cells of the segmenting ovum is primary and not secondary, i.e. in the cleavage the segments do not completely separate from one another. But are we justified in speaking of cells at all in this case? The fully segmented ovum is a syncytium, and there are not and have not been at any stage cell limits. I think the cleavage should be rather described not as segmentation, but a multiplication of the nucleus or centre of force which causes a corresponding readjustment in the density of the network at different parts of the ovum, but no break in continuity.The Gastrula arises by a process of epibole and is at first solid.The endoderm masses at first have no nuclei. Nuclei first appear in them during the progress of the epibole by which the gastrula is formed. I have not been able to determine the origin of these nuclei. They either arise de novo in the endoderm masses or migrate into the latter from the ectoderm. The protoplasmic network at the centre of each endoderm mass is denser than at the periphery, but is without the chromatin granules, so characteristic of a nucleus. But I have described a stage of the nucleus in the fertilised unsegmented ovum in which the chromatin granules are almost entirely absent, and in which the network presents no essential difference from the surrounding network. Again, another in which the nuclear network merges so gradually into the surrounding network, that it is impossible to point to any limit between them. I therefore think it quite possible that this central denser protoplasm in the endoderm masses may give rise to the nucleus which subsequently appears.The gastrula is a syncytium; the ectodermal nuclei are arranged around the periphery of the ovum, while the endodermal nuclei are within. The latter are characterised by their angular shape, and by never presenting the karyokinetic figures characteristic of the ectodermal nuclei. The protoplasm of this syncytium is much vacuolated throughout, but the vacuoles are largest in the centre. These central vacuoles unite and give rise to the gut cavity, which opens to the exterior through a point on the surface where the ectodermal nuclei have always been absent. This opening is the blastopore. The blastoporej until quite late in development, is traversed by protoplasmic strands, which anastomose with similar strands projecting from the protoplasm lining the large central vacuole or gut.The gut of Peripatus arises, therefore, as a vacuole in a multinucleated mass of protoplasm, and the gastrula of Peripatus is a multinucleated mass or syncytium, with absolute continuity of the protoplasm of all parts of the ovum.The Mesoderm.--After the definite formation of the blastopore, an area of protoplasm, placed in the ectodermal layer of the syncytium, and characterised by possessing several nuclei less densely packed together than elsewhere, is distinctly visible in the middle line of the ventral surface just behind the blastopore. This area I have called the polar area. Its nuclei undergo division and give rise to the densely packed mass of nuclei of the primitive streak. A part of it seems to persist for some time in the deeper parts of the primitive streak close to the endoderm.The nuclei of the primitive streak migrate forwards between the ectodermal and endodermal nuclei, and take up their position in the protoplasm intervening between the latter.These rows of nuclei are the mesodermal bands. They soon arrange themselves into groups around a central vacuole, and so give rise to the most conspicuous parts of the mesoblastic somites. I leave the ovum for the present at the commencement of the formation of the somites, merely stating that it is still a syncytium.There are a certain number of facts in the above account which are of general interest and seem to deserve more discussion so far as their relation to processes in other forms are concerned. These are:1. The connection between the intra- and extra-nuclear reticulum.2. The segmentation.3. The origin of the gut as a vacuole.4. The syncytial nature of the embryo.5. The origin of the mesoderm.I propose to consider some of these points at once, and to defer the 5th, to Part 3 of this series.A. The nucleus of the unsegmented ovum and of the early stages of segmentation of the Cape Peripatus are particularly favorable for study, because of their large size and the rapid changes which they undergo. I have not been, able to make out the sequence of these changes, but I hope with more material, which I expect to obtain this year, to be able to communicate some more facts concerning them in a future paper.It is a disputed point as to whether or no the nuclear and extra-nuclear reticulum are continuous. Leydig (12), Stricker (16), Klein (9,10,11), and Heitzmann (5), hold that they are. So far as the nucleus of the early segmentation stage, and of the endoderm of Peripatus is concerned, I am able fully to confirm the views of these observers.The general views I hold with regard to the nucleus are stated on p. 189 and I need not repeat them here. I only desire to point out that the opposite view, viz. that the nucleus is isolated, so far as continuity of protoplasm is concerned, is, from a physiological point of view, very difficult to accept; and I think that the burden of proof rests with him who maintains it. The peculiar lobed structure (PI. XII, fig. 3) of certain stages of the nucleus has been described before by other observers, notably by Balfour in his