52. HISTOLOGICAL PROCEDURES 



EXAMINATION OF EXCISED CHICK BLASTODERMS 



The early blastoderms may be transferred to a watchglass in several drops of Locke's 

 or saline medium (warmed to 38° C), oriented with needles into the proper position and 

 then the medium sucked off by means of a fine bore pipette, while encircling the blasto- 

 derm. In this way the blastoderm will be flattened onto the dry bottom of the watchglass. 

 Immediately add fresh medium (at 38° C. ) so that it flows beneath the blastoderm, lifting 

 it up on the surface tension of fluid. If 0. 001% phenol red (pH indicator) is added to the 

 medium, the slightly purple tinge of the alkaline medium will provide an excellent back- 

 ground for greater clarity of the chick embryo structures, without adding any toxic fac- 

 tor. Such a watchglass may be placed on a warming stage or on an electrically controlled 

 heating stage and the living embryo examined for a considerable period of time. 



THE 24 HOUR CHICK EMBRYO 



If one is interested in general morphology of these early stages it is advisable to mount 

 embryos of the same age in the normal position, and also upside down, so that, in the 

 latter instance, one has a view directly into the intestinal portals. 



Such a mount may be made on a glass slide providing a cut-out in filter paper is made of 

 just such size as to frame the area pellucida and mask out the area opaca. If such a 

 blastoderm is first inverted, pulled up onto the slide (while the slide is submerged in 

 solution), most of the mediuin drained off, and the filter paper frame added, the embryo 

 will remain flat. The yolky margins of the blastoderm will adhere firmly to the filter 

 paper. A cover glass may be added providing its corners are elevated slightly by bits of 

 Permoplast, to prevent crushing. The embryo may now be hydrated with the appropriate 

 medium. It must be remembered, however, that the chick embryo cannot acquire suffi- 

 cient oxygen from any aqueous medium directly, and that it can be "drowned", particu- 

 larly if it has already developed its own circulatory system. The ideal environment is a 

 closed space, completely humidified, with the blastoderm floating on the surface of a 

 nutrient nnedium. 



The later stages may be examined in the manner of any vertebrate form. After about 5 

 days the embryo will take on a definite avian appearance, and it will shortly become 



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