34 FRANK R. LILLIE. 



the frog, may be explained by the simple consideration that the 

 cells of the embryonic tissues proper in the chick are devoid of 

 yolk or other nutriment, and hence are dependent for their sub- 

 sequent growth and differentiation upon the circulation ; whereas 

 the cells of the frog embryo are loaded with food in the form of 

 yolk ; and the extra-embryonic blastoderm of the chick is a diges- 

 tive organ lying on an immense reservoir of food. 



The necessity of circulation for all normal development of the 

 chick embryo beyond the stage of about 33 hours (12-14 somites) 

 at once limits the range of defective embryos capable of develop- 

 ment to those possessing a heart and vitelline circulation. Another 

 limitation arises from the extreme sensitiveness of the embryo to 

 removal of parts of the brain ; although I have made over seventy 

 experiments on the brain, none of the embryos, in which the injury 

 extended back of the optic stalks, has developed for more than 

 about forty-eight hours after the operation. (The operations on 

 the head form a class in themselves and will be discussed in a sepa- 

 rate paper.) The reason for the large number of fatalities in opera- 

 tions in this region is probably not due to any trophic function of 

 the nervous system, at least in stages younger than 72 hours, 

 but either to direct injury to the anterior end of the heart, or to 

 malformations of the amnion consequent on the operation. On 

 the other hand, embryos may survive the destruction of a con- 

 siderable portion of the posterior end, and develop normally for 

 several days as least. All the defective embryos to be described 

 resulted from operations of this kind, performed, with one excep- 

 tion, on embryos in which the tail-bud is just forming after 50- 

 60 hours' incubation. 



A larger or smaller part of the posterior end was destroyed by 

 cauterization. The embryos might bear complete destruction of 

 the posterior end up to the vitelline arteries, provided these were 

 not injured, without any apparent detriment or hindrance to the 

 development of the uninjured parts. If the vitelline arteries 

 were destroyed, the embryo never survived. Fig. i gives a view 

 of an embryo of about the age of those used for operations. In 

 this case 29 somites are formed ; the number was certainly not 

 over 30 at the hour of operation in any case. The somites are 

 continued backward by the undivided segmental plate represent- 



