38 COMPARATIVE ANATOMY 



columnar, forming a perfectly flat plate. The cells on the 

 upper side become flattened and form a relatively thin roof to 

 the still persistent, though much reduced blastocoele. The meso- 

 meres lie side by side in the posterior end of the blastocoele ; 

 the mesoblast bands in front of them diverge from one another 

 and pass forwards and downwards, their anterior ends being 

 widely separated from one another at this stage. 



The flattened embryo now begins to fold up along the 

 antero-posterior axis, and at the same time the anterior and 

 posterior ends are bent over downwards so that the lower 

 surface formed by the large cells becomes concave. At the 

 same time, the small cells on the upper side begin to grow 

 over the large cells all round the edge of the plate. As the 

 plate of large cells at the lower side becomes more and more 

 concave through the folding over of its edges, and as the 

 small cells grow further and further over the infolded cells of 

 the lower layer, an embryo such as is shown in fig. 8, Zf, is 

 produced. It is ovoid in shape, and on its lower side there is 

 a large oval orifice leading into a cavity which is lined by the 

 large cells which at an earlier stage formed the flat lower 

 plate. The cavity is the enteron or primitive gut, and the 

 large orifice leading into it is the blastopore. The lips of the 

 blastopore are still bounded by large cells, but elsewhere the 

 small flattened cells have grown over the large cells and form 

 a nearly complete external investment for the embryo. As a 

 result of the folding up of the lower plate, the blastocoele has 

 almost wholly disappeared, but the mesomeres and the meso- 

 blast bands derived from them lie between the large and the 

 small cells at the sides of the embryo. The three germinal 

 layers are now definitely established. The small outer cells 

 are the epiblast (or ectoderm), the large cells lining the 

 enteron are the hypoblast (or endoderm), and the pole 

 cells with the mesoblastic bands are the mesoblast (or 

 mesoderm). The process by which the hypoblast cells 

 become folded up, so as to enclose a cavity opening to 

 the exterior by the blastopore, is known as a process of 

 gastrulation. 



In the course of further growth the blastopore becomes 

 slit-like through the continued folding in of its lateral walls, 

 and eventually the walls meet and coalesce behind, the coal- 

 escence passing rapidly forward till only a small orifice is left 



