EMBRYO FORMATION IN OTHER GROUPS OF VERTEBRATES 227 



is not at the centre but is eccentrically based. This provides the first marked 

 sign of bilaterality in the developing embryo. Vakaet (1953) has, however, 

 found that the oocytes of the fish Lebistes show a bilateral structure when 

 they are growing in the ovary. It may well be, therefore, that the dorso- 

 ventral plane in the blastoderm is foreshadowed at a much earher stage. 



Figure 11.2 



A, B and C are lateral views showing three stages in the spreading of the 



blastoderm of the trout egg over the yolk. A', B', and C are views on to the 



blastoderm, which in the last two has been cut in order that it may be spread 



out flat. Bl, blastopore; e.a., embryonic area;^.r. germ ring. 



Gastrulation (Reviews: Pasteels 1940, Oppenheimer 1947) takes place 

 by the in-rolling of the margins of the blastoderm. This occurs all round 

 the circumference, but goes on most rapidly at one point, which becomes 

 the site of embryo formation. The endoderm is the first tissue to be in- 

 vaginated. Its presumptive area lies as a thin band in the margin of the 

 blastoderm, much of it already below the surface (Fig. 11. 3). It is prob- 

 ably concentrated mainly in the posterior region from which the embryo 



