app. METHODS OF EMBEYOLOGICAL EESEAECH 575 



organs B, C, etc. are not exactly at the same stage of development — some 

 are less developed some more in the various individuals. Still for practical 

 purposes this is a useful way of indicating roughly the stage of develop- 

 ment. For example early stages in the development of Vertebrates may 

 be defined by giving the number of mesoderm segments which have 

 developed — these being fairly conspicuous structures and definable by a 

 number. A much better system, however, is to use numbered stages 

 defined by the general external form — the first structural feature met with 

 in the examination of an embryo. Keibel has published "normal plates" 

 of the development of various Vertebrate types in which standard stages in 

 development are defined by accurate figures. Unfortunately some of the 

 normal plates are incomplete as regards the earlier stages during segmenta- 

 tion and gastrulation, but wherever the plates extend over the whole period 

 of development they should be made use of by the working embryologist as 

 his standard stages. Where no normal plates exist the embryologist should 

 make it his first business to construct one by carefully working over the 

 external features of development and defining by careful drawing and 

 description a series of stages which he judges to be roughly equidistant. 



The embryology of any animal is an account of the observable changes 

 which take place in its structure from the zygote stage up to the adult. 

 Logically the investigation of its embryology should proceed similarly from 

 zygote to adult but in actual practice it is better to work in the opposite 

 direction — to commence by getting a clear idea of the adult organization 

 and then to work back from the known to the unknown of earlier stages. 



An embryological investigation should commence with a careful study 

 of the entire embryos or larvae at the various stages. Each stage should 

 be examined first alive by transmitted and reflected light, careful note 

 being taken of any movements due to muscular contraction, ciliary action 

 etc. Particular attention should be paid to the arrangement of the 

 blood-vessels, the time of commencement of heart movements, of circulation 

 of the blood and of the appearance of haemoglobin in the corpuscles. The 

 appearance of chromatophores should be noted : the seat of their first 

 appearance and their reactions — whether by changes of form, movement 

 of pigment granules in their protoplasm, or by actual migration — in 

 response to changes in direction or intensity of light. During this phase 

 of the work constant use should be made of the binocular microscope and 

 rough sketches should be made. 



Embryos of each stage should be submitted to the action of various 

 fixing agents and it is important to watch the embryo during the process 

 of fixing, for the fluid as it gradually penetrates the tissues often makes 

 special structures stand out distinctly for a short space of time — to dis- 

 appear again with further penetration. The fully fixed embryo should be 

 subjected to further careful scrutiny by reflected light under the Greenough 

 binocular. To detect small inequalities of the surface it will be found 

 necessary to arrange the lighting carefully. The Kght from an in- 

 candescent gas-mantle may be concentrated by a large condenser and 

 caused to illuminate the embryonic surface in a tangential direction. It 

 is often well to cover the specimen with a little house of opaque cardboard 

 or metal resting on the stage of the microscope and possessing two 

 apertures one in its roof through which the observation is made and one at 

 the side through which light is admitted. The embryo must of course be 



