56 HOW ANIMALS DEVELOP 



are quite normal: they are total and nearly equal 

 A blastula cavity appears in the usual way, and 

 the cavity grows to an enormous size (Fig. 15). The 

 walls are very thin, only one cell thick, except at 

 one place where there is a little lump of cells hanging 

 down inside like a drop or a swarm of bees. This 

 lump is the inner cell mass and eventually develops 

 into the embryo, while all the rest of the thin- walled 

 blastula develops into placenta, etc., and corresponds 

 to the opaque area in the chicken's tgg. 



The endoderm forms by a process which is so 

 simple that it can hardly be called a gastrulation ; 

 the lowest layer of the inner cell mass simply grows 

 out in all directions till it covers the inner surface 

 of the blastula. Sometimes the mesoderm, or at least 

 part of it, forms in a similar way by differentiating 

 in situ from the cells of the middle part of the inner 

 cell mass. But some mesoderm is also formed as it is 

 in chicken. That is to say, it comes from the primitive 

 streak, which appears on the top of the inner cell 

 mass, in the upper layer of cells, which is the 

 ectoderm. Part of the inner cell mass may also split 

 off to form more of the non-embryonic apparatus, 

 but the embryo always passes through a stage when 

 there is a thickened area, the upper layer of which is 

 the ectoderm with the primitive streak, the lowest 

 layer the endoderm, with probably some mesoderm 

 between. It is not known exactly how much meso- 

 derm is formed by the primitive streak, and no 

 staining experiments have been done, so it is im- 

 possible to give anything like an accurate map of 



