40 Growth in length of the Vertebrate Embryo 



morphologically we must look upon the "yolk mass" as being 

 merely the more ventral segment of a fully segmented ovum. 



If we examine the egg of a sparrow just after it has been laid 

 it has in many cases very much the appearance just described. 

 There is however usually a slight cavity below the lowest layer of 

 small cells and the general yolk mass. In the case of the new 

 laid egg of Gallus the cavity is much more evident. 



The blastoderm consists of a definite outer membrane of cells, 

 of practically a single layer in thickness in its central part though 

 many layered round the peripheral area. A distinct cavity, 

 Fig. 23 sg, filled with fluid and containing many more loosely 

 arranged cells underlies the thin central part of the outer membrane. 

 In surface view this central part appears dark owing to the trans- 



lucency of the membrane itself 

 and the cavity into which we are 

 looking; just as a window ap- 

 pears dark when looked into 

 from outside. The thickened peri- 



ap ^l!II!L > ' Uilii ^ pheral part, as it is opaque 



and rests on a solid yolk, looks 

 lighter in a surface view, Fig. 22. 

 These are the areas of the blasto- 



I*ig. 22 after schauinsland. sparrow . 



egg new-laid. derm known as area pellucida 



ap area pellucida; ao area opaca; an <} area opaca respectivelv. In 

 end th thickening of endoderm. . , , . 



section one would always see in 



addition to the cellular membrane, the non-cellular vitelline 

 membrane extending over blastoderm and yolk alike. 



The cavity containing the fluid lying under the area pellucida, 

 is usually called the subgerminal cavity, and it continually in- 

 creases in size by further accumulation of fluid as development 

 proceeds. It is in fact the future gut cavity, and ought more 

 strictly to be called the archenteron. Until recently it has 

 generally been assumed that it arises in the bird's egg just as 

 the corresponding cavity arises in the reptile's egg, by the 

 gradual accumulating of fluid between the small cells of the 

 segmented blastoderm and the large cells forming the unseg- 

 mented yolk. 



Then as the result of the gradual multiplication of the cells 



