228 



PRINCIPLES OF EMBRYOLOGY 



will later arise, but its lateral and anterior extent is very inadequately 

 known, and it may perhaps continue right round the whole margin of 

 the blastoderm. Lying inside it, and the next material to be invaginated, 

 is the mesoderm, which, according to Pasteels' work on the trout, cer- 

 tainly continues round the whole margin. Inside this again, that is to say, 

 towards the centre of the blastoderm, is the presumptive ectoderm. The 

 shape of the presumptive neural plate described by Oppenheimer in 

 Fundulus is quite different from that assigned to it by Pasteels in Salmo, 

 but the overall disposition of the areas is much the same in the two forms 

 and their differences are perhaps such as we might expect from two not 

 very closely related species. 



Fundulu-s 



Figure 11.3 



Trout 



Presumptive maps of the early gastrula (blastoderm) in the dogfish (Sela- 

 chia), and Fundulus and the trout (teleosts). Endoderm, circles; cephaHc 

 endoderm, small crosses; notochord, close dots; mesoderm, spaced dots 

 (somites, horizontal lines); neural tissue, close vertical lines; epidermis, 

 spaced vertical lines. (After data of Vandebroek, Oppenheimer, Luther, 



and Pasteels.) 



While the gastrulation is proceeding, the blastoderm is expanding 

 rapidly, so as eventually to cover the whole yolk. Its margin is often 

 tliickened, forming a structure known as the germ ring. After the expand- 

 ing blastoderm has passed the equator of the egg, the germ ring contracts 

 and acquires a superficial resemblance to a yolk plug before it covers the 

 yolk mass completely. The extra-embryonic part of the blastoderm later 

 becomes vascularised, and forms the yolk-sac by which the embryo 

 absorbs its nutrients from the yolk. While the blastoderm is still expanding 

 and gastrulation continuing at its margin, the embryo begins to appear 

 (Fig. II .2). The connection between the embryo and the thickened germ- 

 ring caused many of the earher embryologists to suppose that the two 

 lateral halves of the embryo had originally been separated, one on each 



