STRUCTURAL PLAN OF HYDRA 257 



as Flemming's fluid. It is then embedded in paraffin, cut into 

 very thin sections with a microtome,arid stained. Acombination 

 of borax-carmine, and picro-nigrosin following corrosive sub- 

 limate, or safranin and light green following Flemming's fluid, 

 gives the best results. But in either case the action of the 

 reagents and the paraffin has such a destructive effect on the 

 tissues that many of the finer details are lost. 



The second method is that of isolation. The animal is 

 placed from five to ten minutes in a very weak mixture of osmic 

 and acetic acids,* and, after being washed for a minute in 

 distilled water, is placed for a least 24 hours in a mixture 

 of equal parts of dilute glycerine and Ranvier's picro-carmine. 

 It is then carefully removed and placed on a glass slide in 

 a fresh drop of dilute glycerine. Slight shaking suffices to 

 separate the cells from one another, or, if they still adhere, the 

 layers may be stripped off with a fine hair mounted in a pen- 

 holder. The coverslip should be put on cautiously and 

 supported by a hair. If the preparation is successful, the 

 individual cells will come apart and can be very satisfactorily 

 studied, as they are stained by the picro-carmine. 



The ectoderm differs in structure in the tentacles, basal disc, 

 and body wall. That of the body wall will be described first. 

 Each cell is somewhat the shape of an inverted cone, the 

 broad end external and the narrow internal end produced into 

 a tapering process which rests upon, or rather is continued into, 

 one or more contractile fibres placed at right angles to the long 

 axis of the cell. (Fig. 55, A.} The contractile fibre is as much as 

 38 mm. long, and lies close against the surface of the mesoglcea 

 parallel to the long axis of the animal's body. It is invested 

 throughout its length by a fine coat of protoplasm, continuous 

 with the protoplasm of the body of the cell. The protoplasmic 

 investment is raised into a number of little prominences, and 

 so looks jagged and indented on its lower surface. The body 

 of the cell is composed of coarsely alveolar protoplasm, the 

 alveolar walls forming a reticulum of denser substance, which, 

 in the lower moiety of the cell, is drawn out into a network of 

 fibrils which may be traced into a connection with the basal 



* Schneider recommends the following mixture : A '02 % solution of 

 osmic acid, i part. A 5 % solution of acetic acid, 4 parts. This mixture 

 gives good results, but macerates rapidly. The tissues should not be left 

 in it longer than four minutes. 



