60 The Development of the Vertebrate Embryo 



from the blastopore and up toward the animal pole. The detached endo- 

 dermal cells move to the middle of the embryo and form a long trough- 

 shaped structure. The edges of the trough move up and around, finally 

 meet, and the trough is converted into a tube. The mesoderm breaks up 

 longitudinally into three masses: a central tube, the notochord (this later 

 disappears and is replaced by the vertebral column), and two lateral 

 Tiasses which later segment and become somites. 



THE CHICK 



At the end of cleavage, the chick blastula is a small cap of cells sev- 

 eral layers thick that sits on top of the yolk. A thin cavity appears within 

 the blastoderm, so that at completion it is a flattened hollow ball. Despite 

 a great deal of descriptive study, it is not clear whether the lower layer 

 of cells simply splits off from the upper layer or whether cells at the edges 

 of the cap move underneath it and join at the center to form the lower 

 layer. In any case, the upper layer of cells is the source of both ectoderm 

 and mesoderm; the lower layer is the endoderm. 



A long narrow groove called the primitive streak appears on the 

 surface of the blastodisc and runs from the approximate center back to- 

 ward one edge. This edge will become the posterior of the embryo and 

 its opposite will be the anterior. Cells stream toward the primitive streak 

 and then turn under to spread out in a sheet between the ectoderm and 

 the endoderm. This intermediate layer is mesoderm and when it is fully 

 formed, gastrulation is complete. Note that the embryo is still only three 

 isolated sheets of cells like a layer cake. Eventually the blastoderm in the 

 anterior and posterior regions and at the sides will get tucked under the 

 embryo. When the two infoldings meet in the middle, the ectoderm will 

 have been transformed from a single sheet of cells into a complete outer 

 covering. The endoderm will also have been tucked under to produce a 

 long hollow tube, the gut. The mesoderm at the same time gives rise to 

 notochord and somites. Figure 30 is a schematic representation of these 

 changes. 



THE SUBSEQUENT DEVELOPMENT 



OF ECTODERM, MESODERM, AND ENDODERM 



An embryo develops in one sense much as a delta-choked river 

 travels to the sea. That is, it starts as one big channel and then splits up 

 into several smaller ones that in turn branch and rebranch. The original 

 embryonic channel is the fertilized egg. The three intermediate channels 

 are ectoderm, mesoderm, and endoderm. The diagram opposite sum- 

 marizes these ramifications. 



