576 EMBRYOLOGY OF THE LOWER VERTEBRATES APP. 



completely submerged in fluid and is preferably contained in a round glass 

 dish with a layer of pitch or black wax on the bottom in which, if necessary, 

 small excavations can be made in which the embryo can rest securely in 

 the desired position. The glass vessel should be rotated slowly during the 

 observations so as to allow of the incidence of the light from different 

 directions. It is important to observe a number, preferably a considerable 

 number, of embryos of the same stage, as owing to individual variation 

 particular features may be much more distinct in some than in others. 



A number of thoroughly typical specimens of each stage should be 

 picked out for further investigation and these should be carefully drawn 

 under the camera lucida, a piece of millimeter scale being placed by the 

 side of the embryo and drawn at the same time so as to form a reliable 

 record as to dimensions. 



At this stage the normal plates should be constructed if not already in 

 existence and the embryos classified in accordance with them. 



For the study of internal structure the great method is that of cutting 

 the embryo into serial sections l but a much older method, that of 

 dissection, should by no means be ignored. Careful dissections made 

 under the Greenough binocular are often extraordinarily instructive. It is 

 advisable to experiment with embryos fixed according to various methods as 

 different methods give different degrees of consistency, opacity etc. Van 

 Beneden and Neyt's fluid will be found in many cases to give very good results. 



In section - cutting a fetish to beware of is excessive thinness of 

 sections. The expert section cutter is liable to become so interested in 

 his feats in accomplishing the preparation of sections of an extraordinary 

 degree of thinness that he is apt to forget that the criterion of good 

 sections is not simply their degree of tenuity but the relation which their 

 thickness bears to the size of the cell-elements of the particular embryo. 

 Thus while in some cases it is of advantage to have sections so thin as 1 /* a 

 or even *5 ft, in other cases, such as segmentation and gastrulation stages 

 of some of the large heavily-yolked holoblastic eggs, the sections should 

 reach as much as 80 p or 100 //, in thickness. 



Before an embryo is cut into sections its soft protoplasm has to be 

 supported by infiltration with some suitable embedding mass. For this 

 purpose the two substances used at the present time are paraffin of high 

 melting-point and celloidin. Of these the first is used frequently alone 

 but the student should realize from the beginning that if he is to 

 obtain reliable results, especially where yolk is present in the embryonic 

 tissues, he must use both methods and control and check tin- results 

 obtained from one by those obtained from the other. 



The process of infiltrating the embryo with paratlin is usually carried 

 out in a hot-water oven heated by oil, gas or electricity and kept at 

 a temperature just ;il>ov- tin- melting-point of the paraffin by a thermostat. 

 The melted paratlin may be contained in small copper pans preferably 

 plated infill'' \\ith silver or nickel. An essential preliminary is a very 

 thorough dehydration followed by a very thorough soaking in tin- clearm-. 

 :ej -lit. To get the best results it is well to take tin- eml.ryo through 



1 A useful guide for beginner "///.-/ l>y '' .l;iniirs..n in i>ir|..-iiatiuii. 



For those who already possess an elementary knowledge of the subject an xrrllent 

 \voj i l.nlli'.s \s-<'> Mi<Tiit,i,, in. 



a 1 M= i .-;'.,.. niillimeter. 



