BIOLOGICAL NOTES

Abstract

THE PRIMARY GERM-LAYERS AND THE ORIGIN OF THE MALE AND FEMALE REPRODUCTIVE ELEMENTS—Prof. Edouard Van Beneden, of Liége, three years ago observed that in the marine hydroid polyp Hydractinia the cells forming the test is and giving rise to spermatozoa, were derived from an ingrowth of the outer of the two primary cell-layers which form the foundation of all higher animal bodies, whilst the ova, he found, were simply cells of the inner primitive layer. This complementary function of the ectoderm and endoderm in Hydractinia led him to frame the hypothesis that throughout the animal kingdom the outer cell-layer is male in function, and the inner cell-layer female. On reviewing the facts known as to the derivation of the sexual cell-elements in various groups of animals, he was able to show a considerable amount of evidence in favour of the view that the testis is always ectodermal and the ovary always endodermal. Though accurate observations in this matter are excessively difficult, and definite knowledge as to the facts, in nearly all cases, is still wanting, yet Prof. Van Beneden's hypothesis was plausible and worthy of full consideration. It has been adopted by Gegenbaur in the last edition of his “Grundriss.”The hydroid polyps consisting, as they do for the most part, of the two primary cell-layers in a very slightly differentiated condition, present the most ready field for the further testing of Van Beneden's hypothesis. The observations of Kleinenberg on Hydra were opposed to it. According to these both the sperm-cells and the egg-cells of Hydra develop from the ectoderm. Mr. J. Ciamician, of Vienna, has made a special study of this question in certain genera of hydroids (Zeitsch. WISS. Zoologie, 1878, part 4), and has published careful drawings in support of his statements. In Tubii-laria mesembryanthemum, assuming the accuracy of Mr. Ciamician's drawings, both female and male reproductive cells develop from a hollow in-growth of the ectoderm (the gonophors of the two sexes being distinct), which at first depresses the endoderm, but is afterwards itself flattened out by the up-growth of the endodermal layer of the spadix. In Eudendrium ramosum the ova develop from cells of the ectoderm, and the sperm-cells from cells of the endoderm, precisely the reverse of the relations detected by Van Beneden in Hydractinia. It seems hardly possible to interpret Mr. Ciamician's drawings of Eudendrium in any other sense than that which he himself adopts; the cell-layers at all stages are as clear in this species as they possibly can be. In the female gonophors of Hydractinia, Van Beneden saw an in-pushing of ectoderm at the apex developed in the same place as the in-pushing at the apex of the male gonophor, from which the sperm-cells developed. Van Beneden interpreted the rudimentary in-pushing in the female gonophor as a survival of a primitive hermaphrodite condition of the gonophors. Ciamician considers, on the contrary, that this ectodermal in-pushing is only the commencement of the formation of a medusa (in fact, the space between umbrella-margin and manubrium), which, instead of being completed, subsides into the condition of a medusoid gonophor. Hence, in place of a constant law of ectoderm being male and endoderm being female, we have in the three genera Tubularia, Hydractinia, and Eudendrium, the following variations respectively:—I, ectoderm male and female; 2, ectoderm male, endoderm female; 3, ectoderm female, endoderm male. The only possible generalisation from these facts is that of Ciamician, viz., that primitively the sexual functions are not assigned exclusively to cells of either layer: ectoderm may produce both male and female elements, and so may endoderm. With increased deve lopment and specialisation of structure, the production of reproductive elements would become limited to particu lar tracts of cells, and these would be necessarily either exclusively endoderm-cells or exclusively ectoderm-cells, but might be either one or the other indifferently even in closely-allied genera; and might be the same or comple mentary for the ovary and testis respectively. Neverthe less it must be admitted that though such indefiniteness in the relation of the sexual glands to the primitive cell-layers might be expected in Ccelentera where the differentiation of the two layers is at its commencement (both layers, for instance, developing nematocysts), yet in the higher groups of the animal kingdom we should be justified in looking for absolute constancy in the deriva tion of ovary and testis respectively from one or other (the same or diverse) of the two cell-layersand —we have not ground for supposing that this derivation would be the same in each of the great phyla until we can show that it is more constant in the Vermes than in the Ccelentera.