580 EMBRYOLOGY OF THE LOWER VERTEBRATES AFP. 



plates are fitted on in a similar manner until the particular organ stands 

 out like a solid model in the mass of plates. 



The same set of drawings may be used for different organs : the clove 

 oil is removed by treating with strong spirit, and the water colour by 

 holding under the tap, and then, after drying, a new organ can be coloured 

 in. By colouring merely the cavity of an organ the relations of the cavity 

 can be displayed as by an injection. When finally done with the drawii 

 are removed by scrubbing with " Monkey brand" soap. 



By this method, after a little practice, reconstructions can be made 

 with great rapidity and accuracy. 



Though less accurate and much more tedious the older method of 

 reconstructing with plates of wax is useful for building up a permanent 

 model. Its use is also indicated where only a single specimen is available. 

 Instead of wax plasticine may be used l which allows of a kind of dissection 

 being made, in as much as particular parts of the model may be bent out 

 of the way to display structures which would otherwise be hidden. 



In investigating the development of the skeleton the cartilage is often 

 found to pass by imperceptible gradations into unmocfified mesenchyme. 

 The absence of sharply denned surfaces in such cases makes. the recon- 

 struction method unreliable and it is advisable to supplement it by 

 subjecting the embryo to treatment with a specific stain w r hich picks out 

 the cartilage while leaving the other tissues uncoloured so that the cleared 

 and transparent specimen may be studied as a whole under the binocular 

 microscope. 



An excellent stain for this purpose is v. Wijhe's Methylene Blue.- 

 The embryo is fixed preferably in '5% watery solution of corrosive 

 sublimate, with 10% formalin added just before use, and preserved in 

 alcohol. When about to be stained it should be treated for a day or two 

 with alcohol containing J% hydrochloric acid care being taken to 

 renew this so long as it develops any yellowness due to traces of iodine. 

 The stain consists of a solution of J% methylene blue in 70% alcohol 

 to which 1% hydrochloric acid has been added some time before use. 

 The embryo is stained for a week and is then treated with 70% alcohol 

 containing ]% hydrochloric acid and renewed several times the first 

 day and thereafter once daily until no more colour comes away. The 

 embryo is now dehydrated, cleared gradually in xylol, passed through 

 stronger and stronger solutions of Canada halsam in xylol, and 

 preserved eventually in balsam so thick as to be solid at ordinary 

 temperatures though liquid at 60 C. 



An excellent method of cleaning small cartilaginous skeletons is to 

 place them amongst I'Yog tadpoles \\hich remove the muscle etc. from the 

 ace of the cartilage by means of their oral com I is 



In re-ard to the gi-neral principles of embryological iVM>aivh it need 



hardly be .-aid that, as in other branche- of science, accuracy of observation 



be lir.-t place. And yet, curiously, accuracy may become a 



f;tult. In tho>e I, ranches of science \\hich are more etlectively under the 



control of mathematics it is well ive<gni/ed that in any type of invest i-a 



t.ion then- i> a limit of probable error of observation due to instrumental 



. imperfection or to disturbing factors of one kind or another 



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