x TECHNICAL DIBECTIONS 513 



however in the first attempt to proceed at once to fix the embryo. 

 An essential preliminary is to remove the true amnion which closely 

 ensheaths the body of the embryo. In doing this it is best to com- 

 mence at the region between the heart and the tip of the head where 

 a couple of fine needles may be used to tear the amnion. Its anterior 

 portion may then be seized with fine forceps and pulled backwards 

 over the embryo's head. The operation is simplified by carrying it 

 out immediately after submerging the embryo in fixing fluid as the 

 action of the fluid makes the amnion slightly opaque and therefore 

 more easily visible. If however corrosive sublimate be the fixing 

 fluid fine splinters of coverslip should be used for dissecting off 

 the amnion unless this is done prior to immersing in the fixing fluid. 

 The embryo should again be carefully studied during the process of 

 fixation, many details becoming particularly distinct before the 

 creature becomes completely opaque. Finally the embryo should be 

 studied, preferably with the binocular, as an opaque object, and then 

 prepared either for section cutting or for mounting whole. 



VII. Six Days. — Open freely into the air-space. Carefully tear 

 away part of its inner wall so as to expose part of the vascular area, 

 great care being taken not to injure the latter. Notice the direction 

 in which the vessels of the vascular area converge : this will indicate 

 the direction in which the embryo is to be found. Work towards 

 the embryo, picking off ■ the shell piece by piece, using blunt 

 forceps. Frequently the escape of the air from the air-space allows 

 the vascular area to sink down and leave a wide space between 

 it and the shell membrane. In other cases however it remains in 

 close contact with the shell membrane and in this event the greatest 

 care must be taken not to injure the vascular area as by doing so 

 the very fluid yolk is allowed to escape and the salt solution rendered 

 so opaque that observation of the embryo in situ is made almost 

 impossible. 



Notice that the allantois has increased much in s'ize, that it has 

 become richly vascular and that it is spreading outwards in a mush- 

 room-like manner underneath the serous membrane. It has already 

 spread so far as to cover the embryo nearly completely. 



It is best now to remove the shell entirely and to examine its 

 contents as they lie submerged in the warm salt solution (as shown 

 in Fig. 242). 



With fine sharp scissors cut through the serous membrane just 

 outside the limit of the allantois, commencing on the dorsal side of 

 the embryo where the allantois is not yet closely applied to the yolk- 

 sac. It is easy to do this owing to the coelomie cavity having spread 

 outwards well beyond the limits of the allantois. The allantois being 

 now no longer flattened out, by its continuity with the serous mem- 

 brane all round, its vesicular character becomes apparent, as well as 

 the difference in character of the vascular network on its proximal 

 and distal walls. The relations of the vascular allantoic stalk to the 

 vascular yolk-stalk should be noted : also the fact that the amnion is 



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