128 BIOLOGICAL LECTURES. 



the embryo, but the segmental folds are comprehensive in ex- 

 tent and cannot depend on these few proto vertebrae. 



The examination of a slightly older embryo, Fig. 2, gives a 



Fig. 2. — Young embryo of Acanthias showing primitive segments. Those numbered i-ii lie in 

 front of the point of origin of the vagus nerve. 



similar picture. The segments are a little better developed, 

 and the broad head-region is roughly marked off from the more 

 slender trunk-region. The segments extend along the margin 

 in pairs. As in the former case, they extend beyond the limit 

 of the axial embryo into the embryonic rim. There are eleven 

 pairs in the broadly expanded head end. It may be determined 

 by tracing them into later stages that the eleventh neuromere 

 is just in front of the place of origin of the front root of the 

 vagus nerve. 



It is obvious that this young animal that is to be hatched 

 from the egg as a vertebrate is now an invertebrate. It ex- 

 hibits an arthromeric condition similar to that in animals of the 

 articulated group. The segments, although faintly expressed, 

 are definite in number and arrangement, and the inference to 

 be drawn from their presence is clear. As Dr. Whitman has 

 said: "This is a stage through which every vertebrate passes 

 on its way from the Qg% to the adult, a stage in which the fish, 

 the amphibian, the reptile, the bird, the beast, and man find 

 a common level, and in which every title to superior rank 

 lies in unexpressed potentialities. But more than this ; for 

 it is here that the vertebrate is an invertebrate and stands 

 beside its prototype, the segmented worm. On the same 



