Chap. XVIII.] THE HYDRA. 341 



(Fig. 99, A., ovm.). Impregnation now takes place by the 

 discharge of spermatozoa from one of the testes of the same 

 individual. Both ovary and testis are ectodermal. 



Development. Cleavage would seem to be holoblastic and 

 regular, the cells protruding and retracting pseudopodial pro- 

 cesses along the cleavage grooves. A morula or mulberry mass 

 is thus formed ; and at the close of segmentation the cells are of 

 two kinds, (a.) prismatic cells, constituting an outer layer, and 

 (b.) polygonally flattened cells forming an inner mass 



On the outer surface of each external cell a thin chitinous 

 plate is, according to the most recent researches, formed, and 

 the plates from adjoining cells fuse together so as to give rise 

 to a continuous pellicle. When this has assumed distinctness, 

 a second is formed in a similar way, and then a third, a fourth, 

 and so on, the various pellicles fusing together to form a thick 

 laminated encasing chitinous shell, which is thus morphologically 

 one of the ectodermal tissues of the embryo. Between this cuticle 

 and the germ an elastic membrane is formed. 



After this a remarkable retrograde metamorphosis is said to 

 take place. The cells undergo histolysis (cell dissolution), their 

 boundaries becoming indistinct and disappearing, and the 

 protoplasm of the cell is said to revert to a primitive unseg- 

 mented condition. Within the protoplasm there is then formed 

 a small excentric cavity, the rudiment of the body-cavity, which 

 gradually increases in size until the germ assumes the condition 

 of a hollow sac, with homogeneous walls. In this state it is set 

 free by the bursting of the external chitinous investment. The 

 walls differentiate into a clear superficial zone (ectoderm) and a 

 darker inner zone (endoderm), and within the outer of these 

 layers neuro-muscular, and then interstitial cells, are differen- 

 tiated. The embryo elongates, and the mouth is suddenly 

 formed as a star-like cleft at one end. Bud-like tentacular 

 processes make their appearance at about the same time. The 

 endoderm cells are now differentiated, the inner elastic membrane 

 is thrown off, and the young hydra is set free. 



Several points seem to call for especial notice in this process 



