o2 THE DEVELOPMENT OF THE CHICK 
marginal cells extend out peripherally and form a short project- 
ing shelf beyond the zone of junction, appearing wedge-shaped 
in section (Figs. 28 A, etc.). This we shall call the margin of 
overgrowth. 
Thus we may distinguish the following zones: (1) margin of 
overgrowth; (2) zone of junction; (3) the inner zone of the germ- 
wall, and (4) the original cellular blastoderm (pellucid area) Fig. 29. 
V. ORIGIN OF THE ECTODERM AND ENTODERM 
The ectoderm and entoderm are the primary germ-layers, 
out of which all organs of the embryo differentiate; hence great 
importance attaches to the mode of their origin. But up to the 
present it has not been possible to decide between three con- 
flicting views. These are: (1) The theory of delamination, viz., 
that the superficial cells of the segmented blastoderm form the 
ectoderm and the deeper cells the entoderm; in other words, that 
the blastoderm splits into the two primary germ-layers. This 
is the oldest view, but it has not lacked support in recent times, 
e.g., by Duval. (2) The theory of invagination, viz., that the 
primary entoderm arises as an ingrowth from the margin of the 
blastoderm. This view, which was supported by Haeckel, Goette, 
Rauber, and some others, brings the mode of gastrulation in the 
bird into line with lower vertebrates. (3) A third and relatively 
recent point of view is that the primary entoderm arises as an 
ingrowth of cells from the germ-wall, more particularly from 
the posterior portion. This view, put forward by Nowack, has 
been adopted in substance by O. Hertwig (Handbuch der vergl. 
u. exp. Entwickelungslehre der Wirbeltiere). 
The reason for the conflict of opinion appears to lie mainly 
in the fact that the critical stages occur prior to laying, and no 
one has investigated a complete series of stages. For this reason 
the subject was reinvestigated in the Zodlogical Laboratory of 
the University of Chicago, by Mr. J. Thomas Patterson, at the 
suggestion of Prof. C. O. Whitman. A very complete series of 
stages of the pigeon’s ovum was studied, with results that are 
consistent in themselves and that agree with the principles of 
formation of the primary germ-layers in the lower vertebrates. 
The author has had the opportunity of following the work step 
by step, and is convinced of its accuracy. It is therefore made 
the basis of the following account: 
