52 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 Zoological 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: 



