The Works of Francis Balfour 2-4
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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.
- The Ovum and Spermatozoon | The Maturation and Impregnation of the Ovum | The Segmentation of the Ovum | Dicyemae and Orthonectidae Dicyema | Porifera | Coelenterata | Platyhelminthes | Rotifera | Mollusca | Polyzoa | Brachiopoda | Chilopoda | Discophora | Gephyrea | Chaetognatha | Nemathelminthes | Tracheata | Crustacea | Pcecilopoda | Echinodermata | Enteropneusta | Bibliography
The Works of Francis Balfour Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.
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Vol II. A Treatise on Comparative Embryology (1885)
Chapter IV. Dicyemae and Orthonectidae Dicyema
Online Editor Dicyemidae - tiny parasites that live within the renal appendages of cephalopods. Orthonectida - a small phylum of parasites of marine invertebrates that are among the simplest of multi-cellular organisms.
The structure and development of these remarkable parasites in the renal organs of the Cephalopoda have recently been greatly elucidated by the researches of E. van Beneden ; and although a male element has not been discovered, yet the embryos originate from bodies which have a close similarity to ordinary ova.
Van Beneden has shewn that Dicyema consists in the adult state of (i) a single layer of ciliated epiblast cells, somewhat modified anteriorly to form a cephalic enlargement; and of (2) one large nucleated hypoblast cell enclosed within the epiblast. There are two kinds of embryo, both developed from germs which originate in the hypoblast cell. The two kinds of embryo arise in individuals of somewhat different forms. The one kind, called by Van Beneden the vermiform embryo, arises in the more elongated and thinner examples of Dicyema which have been named Nematogens. These embryos pass directly into the parent form without metamorphosis.
The second kind of embryo, called infusoriform, is very different from the parent, and has a free existence. Its eventual history is not known. It originates in the shorter and thicker individuals of Dicyema; which have been called Rhombogens.
The Vermiform Embryos. The germs or cells which give rise to the vermiform embryos originate endogenously in the protoplasmic reticulum of the axial hypoblast cell. They appear as small but well-defined spheres, with a minute body in the centre. In these spheres a cortical layer becomes differentiated, which gradually increases in thickness and gives rise to the body of a cell, the nucleus and nucleolus of which are respectively formed from the inner part of the original sphere and the minute central body. These germs can originate in all parts of the hypoblast cell and are frequently very numerous.
The germ when completely formed undergoes a segmentation very similar to that of an ordinary ovum. It divides first into two and then into four approximately equal segments. Of the four segments one, however, remains passive for the remainder of the development. The other three divide and arrange themselves so as partially to enclose in a cup-like fashion the passive cell (fig. 6 1 A). The six cells resulting from their division again divide, giving rise to twelve cells, which nearly enclose the passive cell, leaving only a small aperture at one point. The whole process by which the central cell becomes enclosed is, as E. van Beneden points out, identical with a gastrula formation
, FIG. 61. A. GASTRULA STAGE OF DICYEMA TYPUS.
by epibole, and the space where the central cell is left uncovered is the biastopore. The central cell itself gives Gegenbaur, after E. van
origin to the hypoblast cell of the Beneden -) adult, and the peripheral cells to the epiblast.
By this time the embryo has assumed an oval form, and the blastopore is situated at the pole of the long axis of the oval where the cephalic enlargement is eventually formed.
The subsequent development consists mainly in the closure of the blastopore, and an increase in the number of the epiblast cells. Before the development is completed, and while the embryo is still in the body of the parent, two germs, destined themselves to give rise to fresh embryos, appear in the hypoblast cell, one on each side of the nucleus (fig. 61 B). The embryo continues to elongate, while the anterior cells become converted into the polar cells. Cilia appear simultaneously over the general surface, and the embryo makes its way out of the body of the parent, usually at the cephalic pole, and becomes itself parasitic in the renal organ of the host in which it finds itself.
At the time of birth the embryo may contain a number of germs and sometimes even developing embryos.
Infusoriform Embryos. The infusoriform embryos are capable of living in sea-water and almost certainly lead a free existence. In their most fully developed condition so far known they have the following rather complicated structure (fig. 62 D, E, F, G).
The body is somewhat pyriform, with a blunt extremity which is directed forwards in swimming, and a more pointed extremity directed backwards. The former may be spoken of as the anterior, and the latter as the posterior extremity or tail. At the anterior extremity are situated a pair of refractive bodies (f) which lie above an unpaired organ which may be called the urn.
The structure of the urn, the refractive bodies, and the tail may be dealt with in succession.
The urn consists of three parts: (i) a wall (#), (2) a lid (/), and (3) contents (gr). The wall of the urn is hemispherical in form, and composed of two halves in apposition (fig. F). Its concavity is directed forwards, and in its edge are imbedded a number of rod-like corpuscles which appear as a ring near the surface in a full-face view (fig. D). The lid has the form of a low pyramid with its apex directed outwards. It is made up of four segments (fig. D). The contents of the urn, which completely fill up its cavity, are four polynuclear cells arranged in the form of a cross which appear with low powers as granular bodies (fig. F). They are frequently ejected, apparently at the will of the embryo.
The refractive bodies (r), two in number, one on each side of the middle line, are composed of a material which is not of a fatty nature, and which is passive to the majority of reagents. Each is enveloped in a special capsule, and at times more than one refractive body is present in each capsule. The tail is a conical structure formed of ciliated granular cells.
No plausible guess has been made as to the function either of the urn or of the refractive bodies.
The infusoriform embryos originate from germs, which have however a different origin to the germs of the vermiform embryos. One to five cells appear in the axial hypoblast cell, in a way not clearly made out, and each of them gives rise by an endogenous process to several generations of cells, all of which develop into infusoriform embryos.
FIG. 62. INFUSORIFORM EMBRYO OF DICYEMA.
A. B. C. Three of the later stages in the development.
D. E. F. Three different views of the full-grown larva. D. from the front, E. from the side, and F. from above.
G. side view of urn.
u. wall of urn ; /. lid of urn ; r. refractive bodies ; gr. granular bodies filling the interior of the urn.
The primitive cell is called by Van Beneden a Germogen. In its protoplasm a number of germs first appear endogenously, but the nucleus of the germogen does not assist in their formation. They eventually become detached from the parent cell, around which they are concentrically arranged. A second and then a third generation of germs are formed in the same way, till the whole of the protoplasm of the primitive cell is absorbed in the formation of these germs, and nothing of it remains but the nucleus. The germs so formed are arranged in about three concentric layers, of which the innermost is the youngest. One to five masses of germs may be present in a single Rhombogen. The germs undergo a division, in the course of which their nuclei exhibit very beautifully a spindle modification. In the course of the segmentation the embryo gradually assumes its permanent form, and four of the cells composing it can be distinguished from the remainder by their greater size (fig. 62 A, ;/). The two largest of these give rise to the wall of the urn, and also give origin to four smaller cells (fig. 62 B, gr) which eventually become polynuclear and constitute the four granular cells in the urn. The two other cells become the lid of the urn. The parts of the urn lie at first side by side, but in the course of development the cells which form the wall of the urn travel inwards, and the four granular cells are carried into their concavity. At the same time the cells which form the lid of the urn alter their position so as to overlie the wall of the urn. The two cells immediately above the urn give rise to the refractive bodies (fig. 62 A, B, C, r) and the remainder of the cells of the embryo become the tail (fig. 62 C). The embryo becomes ciliated, and attains its nearly full development before leaving the parental tissues. It usually passes out at the cephalic extremity.
As has already been stated, it is probable that the infusoriform embryos leave the renal organs of their host and lead a free existence. What becomes of them afterwards is not however known, though there can be little doubt that they serve to carry the species to new hosts.
Till the further development of the infusoriform embryo is known it is not possible to arrive at a definite conclusion as to the affinities of this strange parasite. Van Beneden is anxious to form it, on account of its simple organization, into a group between the Protozoa and the Metazoa. It appears however very possible that the simplicity of its organization is the result of a parasitic existence ; a view which receives confirmation from the common occurrence of the process of endogenous cell-formation in the axial hypoblast cell. It has been clearly shewn by Strasburger that endogenous cell-formation is secondarily derived from cell-division ; so that the occurrence of this process in Dicyema probably indicates that the hypoblast was primitively multicellular. It is not improbable that the enigmatical infusoriform embryo may develop into a sexual form, the progeny of which are destined to complete the cycle of development by becoming again parasitic in the renal organ of a Cephalopod.
(117) E. van Beneden. " Recherches sur les Dicyemides." Bull. d. FAcadtmie roy. de Belgique, i" ser. T. XLI. No. 6 and T. XLII. No. 7, 1876. Vide this paper for a full account of the literature.
(118) A. Kolliker. Ueber Dicyema paradoxum den Schmarotzer der Venenanhdnge der Cephalopoden.
(119) Aug. Krohn. "Ueb. d. Vorkommen von Entozoen, etc." Froriep Notizen, vii. 1839.
A number of minute parasites infesting various Nemertines, Turbellarians, and Ophiuroids have recently been studied by Giard and Metschnikoff, the former of whom has placed them in a special group which he calls the Orthonectidae. They were first discovered by W. C. Mclntosh.
In the adult state they are 1 (Metschnikoff) somewhat pear-shaped bodies formed of a kind of plasmodium of cells with irregular lobate processes. In the interior of this body are eggs in all stages of development. In the type observed by Metschnikoff (Intoshia gigas) the ova undergo a regular segmentation, resulting in the formation of a blastosphere in which an inner layer is subsequently formed by delamination. A smaller and a larger kind of embryo are formed ; but all the embryos in each female belong to one type. The larger become females and the smaller males.
The female embryos are ovoid. The outer layer of cells or epiblast becomes ciliated, and divided into nine segments, of which the second is marked off from the remainder by the absence of cilia, and by being provided with refractive corpuscles. The inner layer which surrounds a central cavity, and might be supposed to be the hypoblast, becomes according to Metschnikoff converted into ova.
The male embryos are more elongated than the female, from which they further differ in only having six segments. The cells of the inner layer eventually divide up into spermatozoa.
The larva} probably become free, and while in the free state impregnation would appear to be effected. When the female larvae become parasitic they undergo a metamorphosis, the stages of which have not been observed ; but in the course of which the epiblast cells probably unite into a plasmodium.
The observations of Giard are in several points irreconcilable with those of Metschnikoff, but from the statements of the latter it appears possible that Giard has made two genera from the males and females of one species ; and that Giard's account of an unequal segmentation followed by an epibolic gastrula, in one of his species, has arisen from two segmenting ova temporarily fusing together. Giard has given a description of internal gemmiparous reproduction, upon the accuracy of which doubts have been thrown by Metschnikoff. The affinities of the Orthonectida: are as obscure as those of the Dicyemida? ; though there can be but little doubt that their organization has been much simplified in correlation with their parasitic habits. The origin of the genital products in the axial tissue is a feature they have in common with the Dicyemidae.
1 This at any rate holds true for the type investigated by Metschnikoff. The full history of other forms is not yet known.
(120) Alf. Giard. " Les Orthonectida classe nouv. d. Phylum des Vers." Journal de PAnat. et de la Physiol., Vol. XV. 1879.
(121) El. Metschnikoff. " Zur Naturgeschichte d. Orthonectidae." Zoologischer Anzeiger, No. 40 43, 1879.
[Ch. Julin. " Rech. sur 1'organization et le devel. d'Orthonectides." Arch. BioL Vol. in. 1882.
E. Metschnikoff. " Untersuchungen lib. Orthonectidae." Zeit. f. Wiss. Zoologic, Vol. xxxv. 1881.
For general account of Orthonectidse, vide Spengel. Biolog. Centralblatt, No. 6.]