THE EP1BLAST. 105 



with the exception of a small circular patch at the lower pole 

 (Figs. 52, y, and 54 yp), where alone the white yolk-cells come 

 to the surface. 



This extension of the epiblast occurs all round its margin, 

 and is effected by the addition of cells cut out from the super- 

 ficial layer of yolk-cells. This superficial layer first becomes 

 pigmented, and then divides into, (i) a surface stratum of small 

 epiblast cells, which from the first are similar to the original 

 epiblast cells, and are added on round their margin ; and (ii) a 

 deeper mass of larger and non-pigmented yolk-cells. 



During this extension of the epiblast, the process of cell 

 division has been continuing rapidly in all parts of the embryo. 

 The epiblast now consists of two very definite layers of cells : 

 an outer or epidermic layer (Fig. 51, ee), formed by a single 

 stratum of short columnar cells, which are deeply pigmented, 

 and packed close together side by side ; and an inner or nervous 

 layer (Fig. 51, en), consisting of smaller, more spherical cells, 

 less strongly pigmented than those of the epidermic layer, and 

 arranged two or three deep. The cells that are added on round 

 the margin of the epiblast, during its spreading, are similar in 

 shape and size to the epiblast cells derived from the upper pole 

 of the egg, and, like these, soon become arranged in epidermic 

 and nervous layers. 



3. The Mesenteron. 



The alimentary cavity, or mesenteron, is formed as a narrow 

 slit, opening to the surface at the lower pole of the egg and 

 extending a certain distance into its interior (Fig. 51, bp). The 

 slit rapidly deepens, spreading concentrically with the surface of 

 the egg, and lying near to what will subsequently become the 

 dorsal surface of the embryo ; it is at first exceedingly shallow, 

 its two walls being almost in contact (Figs. 52, 53, t) ; but 

 very shortly, by depression of the lower wall or floor (Figs. 54, 

 55, 56, t), the cavity becomes of considerable size, and forms 

 the alimentary tract of the embryo. 



This slit-like mesenteron was formerly described as arising 

 by a process of invagination, the epiblast cells being said to grow 

 into the interior of the egg to form the wall of the mesenteron 

 cavity. Later investigations have shown that this description is 

 incorrect, and that the cavity is formed, not by invagination from 



