. Elements of Comparative Anatomy. 120 COMPAEATIVE ANATOMY. ectoderm, and are the ova (o). They gradually form a layer of cells placed apparently between the ectoderm and eudoderm, and give to the whole bud the appearance of an ovary. While these processes of differentiation are going on in the endoderm, a growth of cells from the ectoderm at the tip of the bud is extending inwards {A) ; as these cells become separated off from the ectoderm {B), they form a thin lamella, which grows around the ovarian layer, but which has no further function except in another kind of bud. In the male bud, in f
. Elements of Comparative Anatomy. 120 COMPAEATIVE ANATOMY. ectoderm, and are the ova (o). They gradually form a layer of cells placed apparently between the ectoderm and eudoderm, and give to the whole bud the appearance of an ovary. While these processes of differentiation are going on in the endoderm, a growth of cells from the ectoderm at the tip of the bud is extending inwards {A) ; as these cells become separated off from the ectoderm {B), they form a thin lamella, which grows around the ovarian layer, but which has no further function except in another kind of bud. In the male bud, in fact, the ectoderm has the same characters, but the endoderm does not undergo any change, and simply forms a layer of cells, investing the gastric cavity without being differen- tiated into ova. The depressed portion of the ectoderm being developed to a great size, forms by constriction a layer between the ectoderm and endoderm (Pig. 48, A B C), the cells of which give rise, later on, to the morphological elements of the sperm. In this. Fig. 48. Three male generative bnds of Hydractinia echinata. a Testes. Other letters as iu Fig. 47 (after Ed. van Beneden). way the male products of generation arise from the ectoderm, just as the female products are formed from the endoderm. The fact that even in the female buds the ectoderm is depressed, leads us to sup- pose that the buds were prunitively hermaphrodite. It is not yet known how far the generative products have separate origins in the rest of the Acalephse. The possibility of cellular elements having passed from one layer to another at a very early period of develop- ment may account for the fact that the endoderm appears to be the layer in which the products of both sexes are formed. Hydra appears to form an exception, for in it the generative products are formed in external bud-like organs, which are differentiations of the ectoderm. Among the Hydromedusfe we not unfrequently meet with a separation of the sexes, not only mto d
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