l6 LIBBIE H. HYMAN. 



anterior center, although in the chick the two centers are some- 

 what close together at first. In all cases a region previously 

 lower in susceptibility than the anterior center begins to increase 

 in susceptibility and generally eventually surpasses the anterior 

 center in activity for some time. Further criticisms by Adel- 

 mann and Kingsbury of the application of susceptibility data 

 to chordate development have been answered by Child ('25). 



I also wish once more to record my disbelief in the concrescence 

 theory of the formation of the primitive streak. I find no 

 evidence in any of my work with vertebrate embryos of the 

 occurrence of such a process. Neither can I accept the germ 

 ring interpretation of chordate development as expounded for 

 instance in Kellicott's "Chordate Development." The germ ring 

 type of development occurs in certain meroblastic eggs and 

 appears to be simply a method of inclosing the yolk by the 

 blastoderm. The germ ring of the teleost embryo does not 

 appear to be of much more importance in the formation of the 

 embryo than the growing margin of the blastoderm in the chick 

 embryo. The posterior growing point of teleost embryos situated 

 in the germ ring corresponds in my opinion to the dorsal lip of 

 the blastopore of amphibian embryos. It probably appropriates 

 neighboring cells as does also the latter. 



Textbooks of vertebrate embryology should probably be re- 

 vised, emphasizing the origin of the chordate embryo from two 

 centers of activity, a primary center which forms the anterior 

 end of the embryo and a secondary posterior center which grows 

 backwards and forms the remainder of the embryo. The recent 

 beautiful experiments of Spemann and Mangold ('24) establish 

 beyond doubt the importance of this secondary center in the 

 production of the embryo. Whether or not a primitive streak 

 appears in the development of a chordate seems to depend on the 

 time relation between gastrulation and the establishment of the 

 posterior center. If the posterior center arises somewhat tardily- 

 after gastrulation development occurs with the formation of a 

 primitive streak; but where the center is early established a 

 primitive streak is absent from the development since differ- 

 entiation then begins at once. 



Paris ('24) in a study of pigment formation in the Amblystoiua 



