Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/61

Rh of the body substance, more permeable to alimentary matters than the rest, which might be fairly termed a permanent alimentary tract. Moreover, there is much reason for re garding the endoplast and endoplastula as generative organs, while there is, sometimes, a rather complex persis tent system of water vessels. But, on the other hand, this complexity of organisation is different from that observed in the higher animals, inasmuch as the various structures enumerated do not result from the metamor phosis of histogenetic cells, but arise by immediate differ entiation of the finely granular protoplasm of which the body is composed. And, so far, Von Siebold appears to have been fully justified in regarding a ciliated Infusorium as the homologue of a single cell. This is a view which will present no difficulty to those who are familiar with the morphology of the lower plants. The complicated my celium of Mucor Mucedo, for example, is, while young, nothing but a single cell ; and, in Caulerpa, a single undi vided cell grows, without division, into an organism which emulates one of the higher Alga? in the diversity of its parts.  II..

The germ becomes differentiated into histogenetic cells, and these cells become arranged into two sets, the one con stituting the outer wall of the body, or ectoderm, while the other, or endoderm, lies internal to the foregoing, and con stitutes the lining of the alimentary cavity, when, as is usually the case, a distinct alimentary cavity exists. In the embryo, the representatives of these two layers are the epiblast and hypoblast. All the Metazoa, in fact, commence then- existence in the form of an ovum, which is essentially a nucleated cell, sup plemented by more or less nutritive material, or food yelk. The ovum, after impregnation, divides into cleavage masses, or blastomeres, giving rise to a Morula, in the midst of which arises a cavity, the blastoccele (cleavage cavity, " Furchungshohle" of the Germans), which maybe larger or smaller, filled only with fluid, or occupied by food yelk. "\Then it is largest, the blastomeres, disposed in a single layer, form a spheroidal vesicle, enclosing a correspondingly shaped blastocoele. When it is reduced to a minimum, the Morula is an almost solid aggregation of blastomeres, which may be nearly equal in size, or some may be much larger than others, in consequence of having undergone less rapid division. The next stage in the development of the embryo of a Metazoon consists (in all cases except a few parasitic anenterous forms) in the conversion of the Morula into a body having a digestive cavity, or a Gastrula. The animals in which the embryo takes on the form of a Gastrula, may be termed, as Haeckel has proposed, Gastrcece. The conversion of the Morula into the Gastrula may take place in several ways. In the simplest, the Morula being composed of equal or nearly equal blastomeres, more or less completely converted into cells, these differentiate themselves into an outer layer, the epiblast, investing the remaining cells, which constitute the hypoblast. The central cells of the hypoblast next diverge and give rise to a space filled with fluid, the alimentary cavity, which opens at one end, and thus gives rise to the Gastrula. This is the process generally observed in Porifera, Ccelenterata, Turbellaria, Trematoda, and Nematoidea. In a second class of cases, the Morula becomes converted into blastomeres of unequal sizes, a small and a large set. The smaller rapidly become converted into cells, and invest the larger and any remains of the food yelk, as a blastoderm, which at first represents only the epiblast of the former case. The hypoblast arises either from the epiblast thus formed, or from the included larger blastomeres. This is the process observed in certain Turbellaria, in the Ctenophora, in the Oligochoeta and Hirudinea, in the Artliropoda, and in most Vertebrata. In a third group of instances, the Morula, whether con sisting of equal or unequal blastomeres, becomes spheroidal, and encloses a correspondingly shaped blastoccele. One part of the wall of this vesicular Morula then becomes invaginated, and gives rise to the alimentary cavity, with the hypoblast which limits it. This process has been observed in the Ch&tognatha, Echinodermata, Gephyrea, polychsetous Annelida, Enter opneusta, Brachiopoda ; in most Mollusca ; in Amphioxus ; and, slightly modified, in Petromyzon and in the Amphibia. These various modes in. which the two primary layers of the germ may be developed shade off into one another, and do not affect the essence of the process, which is the segregation of one set of cells to form the external covering of the body, and of another set to constitute the lining of the alimentary canal. In whatever manner the Gastnda is formed, and what ever be its shape when its alimentary cavity is complete, one of two things happens to it. It becomes provided with many ingestive apertures, distinct from that first formed ; or with only one, which may or may not be distinct from the first aperture. The former division comprises only the Sponges (Porifera or Spongida) in which, as the remark able researches of Haeckel have shown, the walls of the deeply cup-shaped Gastrula become perforated by the numer- rous inhalent ostioles, while the primitive opening serves as the exhalent aperture. These may be termed the Metazoa polystomata. The latter division includes all the remaining forms, which may be grouped together as Metazoa monostomata. Among these, two primary groups are distinguishable, of which the second exhibits an advance in organisation upon the first. In the first, the aperture of the Gastrula be comes the permanent mouth (Archceostomata). In the second, the permanent mouth is a secondary perforation of the body wall (Deuterostomata). 1. It is now well established that the aperture of tho Gastrula becomes the oral aperture of the adult in the Coslenterata, which group includes animals differing much in grade of organisation, from the simple Hydra to the complex Ctenophore, but all manifestly exhibiting varia tions of one fundamental type. Parallel with these may be ranged an assemblage composed of the Turbellaria, Kotifera, and Trematoda, which are associated together by the closest resemblances of structure, and which present an even greater range in grade of organisation than the Coslenterata. The lower Rhabdoc&la come very close to the Infusoria (as close as the multicellular to the unicellular Algae), and are but little superior to Hydra in the degree of their organic differentiation ; while, in the Trematoda, the laud Planarice, and the Nemertidce, we have animals which attain a considerable complexity, and in the case of many Trematoda and of Linens (Pilidium) undergo remarkable metamorphoses. As a cognate group, the Nematoidea may be enumerated, extremely simple in their lowest forms, considerably differentiated in the higher, and connected with the Turbellaria by such forms as Polygordius. The Oligochaia and the Hirudinea also belong to this division of Scolecimorpha, but they differ from the foregoing in the development of a segmented rnesoblast. In the Ccelenterata, Nematoidea, Turbellaria, Trematoda, and Rotifera, the mode of origin of the cells which lie ) between the epiblast and the hypoblast, constitute the meso- / blast, and give rise to the connective tissue and muscles of the body wall and of that of the intestine, is not precisely known. They may take their origin in the epiblast, or in the hypoblast, or in both. But in the Oligoch&ta and the Hirudinea, after the epiblast and hypoblast are difieren-