78 THE DEVELOPMENT OF THE CHICK 
The mode of origin of the mesoderm of birds has been a very puzzling 
question as is proved by the numerous views that have been in vogue 
from time to time. One of the earliest views was that the mesoderm 
arose by splitting of the primary entoderm (Remak). This view sur- 
vives in part even at the present time (mesoblast of the opaque area). 
Balfour believed that the mesoblast in the region of the embryo “ ori- 
ginates as two lateral plates split off from the primitive hypoblast,” and 
that the primitive streak mesoblast is extra-embryonic, or at most enters 
into the formation of mesoblast of the extreme hind end of the embryo 
(allantois mesoblast in part). This view is found in the “ Elements of 
Embryology” of Foster and Balfour. A third view, now of historical 
interest only, was that the mesoblast cells arose peripherally and mi- 
grated between the two primary germ-layers (Peremeschko, Goette). 
The latter author even attempted to derive the primitive streak from 
an aggregation of such inwandering cells. The view that the primitive 
streak arises as a thickening of the ectoderm and that it is the source 
of all the mesoderm was first stated by Iolliker, and has been accepted 
by Hertwig, Rabl, and many others. It may, indeed, be regarded as 
definitely established for the embryonic mesoblast. Others, however, 
believe with His that the mesoblast of the opaque area arises by delam- 
ination from the germ-wall; this question is discussed beyond. It should 
also be noted that it is probable that the primitive embryonic mesoblast 
is supplemented in certain regions at later stages by cells proliferated 
from both entoderm and ectoderm, particularly in the region of the 
head. (See pp. 116, 117.) 
In early stages oi the primitive streak the mesoblast cells 
are relatively sparse and bear every appearance of migrating 
separately. But as the ingrowth progresses and the cells become 
more numerous, the mesoderm becomes converted into coherent 
plates. These are wedge-shaped, the central broad ends fused 
with the primitive streak and the narrow margins extending 
laterally (Figs. 40 A, B,C). They soon overlap the margin of 
the opaque area and thus is produced a three-layered portion of 
Fig. 40. — Three transverse sections of a late stage (corresponding to about 
Fig. 44 B), through the head-process and primitive streak of a chick embryo. 
A. Near the hind end of the head-process. 
B. Through the primitive pit. 
©. A short distance behind the center of the primitive streak. The region 
between the lines A~A and B~-B is represented under a high magnification 
in Fig. 41. 
Bl. I.. Blood island. coel. Mes., Coelomie mesoblast. Eet., Eetoderm. 
Ent., Entoderm. G.W., Germ-wall. med. pl., Medullary plate. Mes., Meso- 
derm. N’ch., Notochord. pr. f., Primitive fold. pr. gr., Primitive groove. 
pr. p.. Primitive pit. 
