620 The American Naturalist. [July, 
stance in Pristiurus, Rabl, 44, 116-129, the notochord may be 
differentiated while the mesoderm is still developing in the em- 
bryonic rim. The precocious changes in the embryonic rim de- 
mand especial attention when the origin of the mesoderm is dis- 
cussed. The ectoderm, as soon as it becomes one-layered, con- 
sists of high cylinder cells. As development progresses the ecto- 
derm thins out except at either side of the axial line. The meso- 
derm arises from the entoderm, close to the ectental line, and is 
there quite thick, but as it stretches away it thins out. ' Now, if 
it be remembered that the ectental IIne becomes the axial line 
when concrescence takes place it is evident that this mesodermic 
thickening of the entoderm is in reality axial thickening, and 
when concrescence takes place it fuses with the corresponding 
thickening of the opposite side and constitutes an actual axial 
thickening or true primitive streak ; but inelasmobranchs as soon 
as the anterior axial structures have concresced we find by preco- 
cious development that the notochord and medullary groove ap- 
pears ; now, as I have shown elsewhere, the appearance of these 
structures causes the division of the axial mesoderm into com- 
pletely separated right and left portions. It is only by keeping 
the process of concrescence and the precocious development of 
the parts constantly in mind that we can understand the develop- 
ment in elasmobranchs or compare it rightly with that of other 
types. From what has been said it is clear that a section of the 
blastodermic rim from which the mesoderm was just growing out 
would correspond to half a section of, say, a bird’s ovum through 
the primitive streak, and upon comparison it will be found that all 
the essential relations are identical. 
The structure of the primitive streak in birds has been re- 
peatedly investigated and the subject of much discussion. As 
the observations of Duval, 77, 18, appear to me the most 
thorough, and as my own preparations have enabled me to 
confirm many of his statements, I follow in the main that author. 
Duvals statements have also, been verified in many essential 
points. by Zumstein, 69. Other important authorities to be con- 
sulted are Kölliker in both his text-books; His 25, 26, 29, ete, 
