CHAPTER XVI 



SOME EMBRYOLOGICAL METHODS: THE CHICK, SECTIONS 



AND "IN TOTO" MOUNTS; AMPHIBIA; FISH; 



MAMMALS; OTHER FORMS 



THE CHICK 



The hen or an artificial incubator is necessary. In many ways 

 the latter is more convenient as it may be kept in the laboratory 

 and is ready at all seasons of the year. There are many 

 kinds of good incubators on the market at present which may be 

 had for a small sum. 



Whatever method of incubation is employed the eggs must be 

 fresh and must not have been subjected to rough handling. The 

 date and hour at which incubation is to begin should be written 

 on the shell of each egg in ink. If late stages of development 

 are desired the egg must be turned every few days. All products 

 of combustion from the lamp or burner should be kept from the 

 eggs and the supply of fresh air and moisture carefully main- 

 tained. The temperature should be maintained at 38 C. 

 (100.4 F.). Should it rise above 40 C. embryos will be 

 destroyed. 



Prepare at least 5 embryos as directed in the practical exer- 

 cise ; 2 for in toto preparations and 3 for sections. 



1. Place an egg which has been incubated for between 46 

 and 54 hours, while it is yet warm, in a vessel which contains 

 sufficient normal saline warmed to 38 C. to cover the egg. In 

 the chick the embryo always makes its appearance as a germinal 

 disc or cicatricula, as it is termed, situated on one side of the 

 yolk, which is the real egg of the hen, the white being simply a 

 nutritive mass added in the oviduct. This disc or blastoderm in 

 the early stages of incubation always turns uppermost no matter 

 in what position the egg may be placed. Moreover, it has been 

 found that the embryo in nearly every instance lies in such a 

 position that when the blunt end of the egg is toward the left, 



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