618 The American Naturalist. [July, 
Structure of the primitive streak —For our purposes it is con- 
venient to give 1°,a general comparative account; 2°, a more 
detailed description of the mammalian ovum up to the stage with 
completed primitive streak. 
1°. Vertebrate primitive streak —It is advisable to begin with 
the consideration of the arrangement as we find it in eggs of 
marsipobranchs, ganoids, and amphibians, since these eggs are 
probably more primitive in their mode of development than those 
of other vertebrates. The points of most importance in my 
judgment are illustrated in Fig.11, A and B. In A, we have a 
section through the middle portion of a partly-formed primitive 
tions of axolotl eggs: after Bellonci. A, frontal section somewhat an- 
FIG. 11.—Sec 
terior to the blastopore, from an egg in which the archenteron was partly formed, but the 
anus of Rusconi not delimiated. B, frontal section of an older ovum with well marked 
but anus of Rusconi; the section passes just in front of the blastopore. Æc, ecto- 
derm ; Zn, entoderm ; Mes, Mesoderm ; Ae, archenteric cavity ; Pr, primitive streak. 
streak of an axolotl, the streak still requiring considerable additions 
at its hinder end before attaining its full length ; the archenteric 
cavity, Ae, is a large space bounded above by an epithelium, ez, 
and below by the large mass of yolk-cells; the two-layered 
ectoderm, Ec, everywhere bounds the section; above the archen- 
teron and below the ectoderm lies the accumulation of cells con- 
stituting the primitive streak, pr ; the lateral prolongations, Mes, 
of the streak represent the commencing mesodermic outgrowths ; 
whether the mesoderm grows out from the primitive streak, and 
