448 



APPLIED BIOLOGY 



Careful observation of the " yolk" of a bird's egg will dis- 

 close a white spot on its upper surface. To see this, lay a 

 fresh egg on some cotton or sawdust, 

 carefully chip the shell with forceps, 

 or cut with scissors, making a hole 

 about one inch in diameter. This 

 white spot is the germ-disc from 

 which the embryo will develop ; 

 and all the other material inside 

 the yolk-membrane is food. Hen's 

 eggs kept under sitting-hens, or in 

 incubators, for 15, 24, 36, 48, and 72 

 hours, and then opened (as directed 

 above) will show a series of stages 

 in the growth of the embryo from 

 the germ-disc. Such stages are 

 FIG. 152. Germ-disc with sometimes removed from eggs and 

 chick embryo after about pre pared for microscopic examina- 



36 hours incubation, hd, . T/ . . 



head forming ; ng, neural tion. If Such preparations are avail- 

 groove (first appearance a bl e , they should be examined. 



of nervous system). (From ^ , . , , , , , 



Marshall.) Eggs which have been incubated 



for several days show the embryo 



surrounded by membranes in part of which there are blood- 

 capillaries. The 

 blood in these is 

 pumped by the em- 

 bryo's heart, which 

 begins to beat on the 

 second day of incuba- 

 tion. The purpose of 

 circulating blood in 

 the blood-vessels of 

 membranes outside 

 the embryo's body is FlG A 153 ' A ' chi f e K mb 7? of , about . f ? re n 



. . J days. B, one of about ten days with all 



(1) Obtaining food organs formed. (From Parker and Haswell.) 



