28 LIBBIE H. HYMAN. 



development, as for instance the area vasculosa, was not de- 

 termined in my experiments and hence it cannot be stated 

 definitely that the same explanation applies to such anomalies. 

 However, it seems very probable that such is the case. The 

 heart is treated in a separate paper which follows this one. 



Finally it should be pointed out that both of two paired 

 structures or the two halves of a median structure are not 

 necessarily equally susceptible; consequently asymmetrical ab- 

 normalities are possible since, for instance, one eye may be 

 slightly more susceptible than the other. 



IV. SUMMARY. 



1 . Differences in the time of death of different regions of chick 

 embryos dying in toxic solutions are described. Chick embryos 

 of various stages from the primitive streak through the third 

 day of development were studied. In primitive streak and head 

 process stages, there is a simple death gradient from the anterior 

 to the posterior end of the embryonic axis. From the medullary 

 plate stage on there are two regions of high susceptibility the 

 anterior end of the axis and the growing point of the primitive 

 streak; from these two regions death progresses towards the 

 middle of the embryo. Other structures showing high sus- 

 ceptibility at certain times are: the special sense organs, the 

 limb buds, and all places where unions, folds, bendings, etc., 

 are about to occur. A more detailed summary is given on 

 p. IT. 



2. Application of the results to normal development leads to 

 the following suggestions, susceptibility or time of death being 

 regarded as roughly proportional to metabolic rate : 



a. There is a gradation in rate of metabolic activity along the 

 primitive streak in an antero-posterior direction. This probably 

 determines the cephalo-caudal progress of development. 



b. The head process is probably merely a further development 

 of the anterior end of the primitive streak. 



c. The primitive streak (excluding the head process) forms all 

 of the embryo from the first somite posteriorly. 



d. Such developmental processes as the union of paired 

 primordia, formation of bends and flexures, occurrence and 



