68 TIIK AMKUUAN MONTHLY [Muivh, 



hodv. l>v coMslanl ailiiition of colls, changes its external shape 

 from a sphere tt» a tube, ami one end jjrows by degrees into the 

 toiiu ol H\ dra. with nianuhiinni and tentacles (lig. 14). Mean- 

 time tile endoilernial cells struggle lor life ; some are devoured hv 

 others, and the tooil thev vield 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. covereil with a tough 

 secretion, remains dormant iluring the winter, its development 

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

 people the pond, its parent having succumbed to the severitN' of 

 the winter. 



(In Litnttochniriufn . a fVesh-watcr animal allied to Hydra, there 

 is a true medusa (see Atn. Mo. Micro. Jul .^ vol. iv. j). 223), but 

 in Hydra the gonails are understood to represent degenerate me- 

 tlusa stages.) 



Explanation of the Figures of Hvdha ki'sca. 



ISKB I'KONTIbriBCK ) 



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

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

 spermarv (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, aftei Kleincnberg.* showing the 

 contractile processes next the supporting lamella. 



Fig. 5. Section of body wall, partly after Schultze (E. Britt., 

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

 the included ttcniatocysts and glandular cells, and the large 

 vacuolated endoderm cells with amouboid and flagellated outer 

 ends and engulphed particles ; si, supporting lamella. 



F^ig. 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. Stages a, b, c, d, e, in the development of spermato- 

 zoon (after Kleincnberg). 



Fig. S. Egg after fertilization. 



Figs. 9, 10, and 1 1. Successive stages of segmentation. 



* See Gegcnbaur, Comp. Aral., p. 30; also E. Britt., ix, p. 549. 



