52 THE DEVELOPMENT OF THE CHICK 



the external margin extends out peripherally and forms a short 

 projecting shelf, 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 with the yolk (syncytial germ- 

 wall); (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 until recently 

 it was not possible, in the case of the chick, to decide between 

 three conflicting views. These are: (1) The theory of delamina- 

 tion, 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 conflict of opinion was due to the fact that the critical 

 stages occur prior to laying, and no one had investigated a com- 

 plete series of stages until recently. The investigations of J. T. 

 Patterson on the pigeon have, however, cleared the matter up. 

 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 first step in the process of gastrulation, or formation of 



