68 THE AMERICAN MONTHLY [March, 



body, by constant addition of cells, changes its external shape 

 from a sphere to a tube, and one end grows by degrees into the 

 form of Hydra, with manubrium and tentacles (fig. 14). Mean- 

 time the endodermal cells struggle for life ; some are devoured by 

 others, and the food they yield nourishes the whole colony. 

 Their removal leaves a space in the centre, and the mouth being 

 formed, food is captured by the tentacles, forced into this space, 

 and the endoderm cells receive supplies, which relieve them of 

 the necessity of cannibalism. The course of this history of Hydra 

 receives usually an interruption. The first part of the develop- 

 ment takes place in the autumn; the egg. covered with a tough 

 secretion, remains dormant during the winter, its development 

 being renewed by the warmth of spring, when the young re- 

 people the pond, its parent having succumbed to the severity of 

 the winter. 



(In Limjwcladriurn. a fresh-watei' animal allied to Hydra, there 

 is a true medusa (see Ati}. Mo. Micro. Jnl.^ vol. iv. p. 223), but 

 in Hydra the gonads are understood to represent degenerate me- 

 dusa stages.) 



Explanation of the Figures of Hydra fusca. 



(see pkontispiece ) 



Fig. I. Surface view of partly contracted specimen, showing 

 bud. (b), tentacles (ten), manubrium (mn), ovary (ov), and 

 spermary (sp) . 



Fig. 2. Diagrammatic longitudinal section, ectoderm cells not 

 shown (ec) ; endoderm, its columnar cells granular from food in 

 ends toward stomach cavity, running up into the tentacles and 

 out into the bud ; supporting lamella (si). Note the absence of 

 any infolding of the ectoderm to form a throat (compare Sea- 

 anemone) . 



Fig. 3. Cross-section of one-half of body from nature, show- 

 ing the smaller ectoderm cells and larger endoderm cells, granu- 

 lar at the end next the cavity and the supporting layer. 



Fig. 4. Isolated ectoderm cells, after Kleinenberg,* showing the 

 contractile processes next the supporting lamella. 



Fig. ^. Section of body wall, partly after Schultze (E. Britt., 

 ix, p. 549), showing the ectoderm with the cuticular border and 

 the included nematocysts and glandular cells, and the large 

 vacuolated endoderm cells with amoeboid and flagellated outer 

 ends and engulphed particles ; si, supporting lamella. 



Fig. 6. Ovum ready for development, showing the pseudo- 

 podial processes and the granular deuteroplasm and the nucleus 

 (after Balfour, Comp. Emb., i, p. 17). 



Fig. 7. wStages a, b, c, d, e, in the development of spermato- 

 zoon (after Kleinenberg). 



Fig. 8. Egg after fertilization. 



Figs. 9, 10, and 11. Successive stages of segmentation. 



*See Gegenbaur, Comp. Arat., p. 30; also E. Britt., ix, p. 549. 



