120 



COMPARATIVE ANATOMY, 



ectoderm, and are the ova (o). They gradually form a layer of cells 

 placed apparently between the ectoderm and endoderm, 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 (Fig. 48, ABC), the cells of which give 

 rise, later on, to the morphological elements of the sperm. In this 



B 



Fig. 48. Three male generative buds of Hydractinia echiuata. a Testes. Other 

 letters as in Fig. 47 (after Ed. van Benedeu). 



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 primitively 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 Hydromedusas we not unfrequently 

 meet with a separation of the sexes, not only into different persons, 

 but even into different colonies ; in the Siphonophora hermaphrodite 

 colonies only are found as the rule, but there are exceptions to this. 

 The generative products give rise to more or less considerable swell- 



