270 STUDY OF YOUNG CHICK EMBRYOS. 



In this work two stages of the chick are especially studied. The first is nor- 

 mally produced after about forty-six hours' incubation. The embryo should 

 have about twenty-eight segments and three gill clefts showing externally. Em- 

 bryos a little less or a little more developed are almost equally serviceable. The 

 second stage studied is that of a chick with seven segments, which is normal 

 after about twenty-seven hours' incubation. The student will find it advan- 

 tageous to begin his studies with older stages, as these can be more easily manip- 

 ulated. 



Removing the Embryo. — Before the ^%g is opened a basin should be prepared 

 and filled with normal salt solution warmed to about 40° C. (104° F,). The 

 basin should be large enough to permit the entire 0:%% to be submerged in it. 



Take the ^g% warm from the incubator or the hen ; allow it to rest quietly in 

 one position for two or three minutes before opening it. This is in order to insure 

 that the side of the yolk which contains the embryo is turned uppermost. After 

 an ^%% is disturbed the yolk will turn and resume its normal position, for which 

 but a short time is necessary. The ^%^ may now be held in one hand, the shell 

 cracked and the pieces of the shell above the yolk be removed with forceps, mak- 

 ing a hole about an inch in diameter. The inner ^gg membrane may be removed 

 with the shell. If any of the white of the ^%g tends to overflow, it should be 

 immediately snipped off with a pair of scissors, otherwise it will cause the yolk 

 to roll over, thus concealing the embryo. 



The embryo and germinal area are now to be examined with the naked eye, 

 or, better, with a hand lens. The student will detect very easily the area pellu- 

 cida, which lies at right angles to the long axis of the ^%g, and also see in the middle 

 of the area a long whitish streak, which marks the anlage of the embryo. Around 

 the area pellucida can be seen the mottled vascular area which will vary in ap- 

 pearance according as the development of the blood-vessels and blood-islands is 

 more or less advanced. The area vasculosa is a portion of the larger area opaca 

 which merges at its periphery into the general yolk. In embryos of the second 

 half of the second day, thirty-six to forty-eight hours, the contraction of the heart 

 can be readily seen, and usually the outlines of the head of the embryo may be 

 made out. The germinal area is now to be separated from the rest of the yolk. 

 To accomplish this, plunge one blade of a pair of sharp .. *-^cors into the yolk a 

 little beyond the edge of the vascular area, and cut rapidly around until a cir- 

 cular incision has been completed ; then take a flat spatula and plunge it boldly 

 into the yolk at a depth of perhaps an eighth of an inch underneath the embryo. 

 Next lift out the embryo together with the yolk and the overlying white of the 

 ^%g, steady it a little if necessary on the spatula with a pair of forceps or needle, 

 and transfer it rapidly to the dish of warm salt solution. With a pair of fine 



