4 LIBBIE H. HYMAN. 



The drawings illustrating the death of chick embryos have been 

 simplified and diagrammatized as much as possible. They are 

 made from free-hand sketches drawn while watching the dis- 

 integration. Close stippling or solid outlines represent intact 

 structures; open stippling dead and disintegrated structures. 



2. Stages before the Appearance of the Typical Primitive Streak. 

 -Technical difficulties have rendered it impossible to reach any 

 satisfactory conclusions concerning very early stages. It is 

 exceedingly difficult, practically impossible, to remove such early 

 blastoderms without inflicting some injury upon them. Since 

 every injury becomes the site of increased susceptibility and since 

 injured places cannot be detected with certainty owing to the 

 delicate texture of the blastoderm, disintegration changes ob- 

 served in such early blastoderms cannot be regarded as conclusive 

 evidence of innate metabolic differences. 



Dr. Leigh Hoadley kindly dissected off several early blasto- 

 derms (seven to ten hours incubation) for me but in no case 

 were we convinced that they had been removed uninjured. In 

 such early stages there is generally visible a central area which is 

 slightly more opaque than the remainder of the germinal disk. 

 This opaque area is designated by Hoadley ('260) as the "broad 

 primitive streak"; its appearance is well shown in his figures. 

 It is a somewhat cone-shaped thickening extending from the 

 center to the posterior margin of the blastoderm. In all cases 

 in which disintegration changes were clearly seen in these early 

 blastoderms this opaque region was decidedly more susceptible 

 than the remainder of the blastoderm and there was some 

 evidence that the death changes progressed in it from its anterior 

 to its posterior end. 



j. Stage of the Typical Primitive Streak. This was the earliest 

 stage on which conclusive observations were made. It about 

 corresponds to Fig. 44 A, p. 87, of Lillie's text. 1 At this time 

 (about twelve to fifteen hours of incubation), the area pellucida is 

 pear-shaped and the primitive streak is an elongated opaque 

 thickening extending from near or anterior to the center of the 

 blastoderm to its posterior margin. The disintegration of the 

 primitive streak is illustrated in Figs. I to 3. Death changes 



1 Lillie, "Development of the Chick," 2d edition, 1919, Henry Holt & Co. 



