514 The American Naturalist. [Juue, 
nately the few authors who have investigated the primitive streak 
in mammals have failed to consider the problem, as the discover- 
ies of His and his followers have rendered it necessary to con- 
sider it, namely, as the question: How is concrescence modified 
in mammals? It is therefore not surprising that we lack the 
necessary information. The organization of the mammalian 
primitive streak is described below. Provisionally, at least, we 
must assume that the streak follows the same developmental 
type in the highest as in the lower vertebrates. 
Concrescence, Summary.—tThe evidence that concrescence is the 
typical means of forming the primitive streak in vertebrates is : (1) 
detailed and conclusive observations upon elasmobranchs, teleosts 
and birds ; (2) exact and extensive observations on marsipobranchs, 
ganoids and amphibians, which concord with the theory of con- 
crescence ; (3) a great probability of its occurrence in reptiles, 
owing to the similarity of their development with that of birds; 
(4) a probability of its occurrence in mammals, because of the 
resemblance in the growth and structure of the primitive streak 
to that in other vertebrates. It seems to me therefore not pre- 
mature to draw the generalization that the vertebrate primitive 
streak is formed by the growing together in the axial line of the 
future embryo of the two halves of the ectental line. 
The development of the primitive streak may be described in 
general terms as follows :—-At the close of segmentation the edge 
of the primitive blastoderm separates into two parts; one part 
(the anterior), as the blastoderm expands, spreads over the yolk, 
gradually covering it with ectoderm ; the other part (the posterior) 
forms the primitive streak; it has in its centre one fixed point, 
consequently when the blastoderm expands the two halves of the 
posterior part of the ectental line are brought together, and 
gradually unite (concresce) along a line running from the fixed 
point radially backwards as regards the blastoderm. Conse- 
quently the segmentation cavity which is underneath the primi- 
tive blastoderm lies in front of the developing archenteron. 
While this goes on, cells grow out from the concrescing part of 
the ectental line into the space between the ectoderm and ento-’ 
derm (or yolk); underneath the line of junction a cavity is 
