CHAPTER XV 



THE BLASTOPORE 



The blastopore is one of the most important structures in 

 development, for as a result of the processes which are entailed 

 in its formation the fundamental architecture of the future 

 embryo is laid down. Further, experimental investigations 

 have shown that the region of the dorsal lip of the blastopore 

 (which is the first part of the blastopore to develop) is 

 responsible for organising the embryo. That is to say that it 

 determines the place of formation of the various organs, and 

 is necessary for the start of the processes of differentiation. 

 The blastopore itself introduces the first differentiation (after 

 the establishment of the axis of the egg in the ovary, and of the 

 plane of bilateral symmetry by the point of entrance of the 

 sperm) in that it converts the single-layered hollow ball 

 (blastula) into the double-layered bowl (gastrula). In those 

 animals where the relation of the sperm's entrance point to 

 the blastopore is known (amphibia), it is found that the dorsal 

 lip of the blastopore arises opposite the sperm-entrance point, 

 and marks the dorsal side of the future embryo. 



In the development of the dogfish, the egg contains so much 

 yolk that cleavage is incomplete or meroblastic, and a disc of 

 cells or blastoderm is formed lying on the top of the yolk. 

 Now the important point to notice is that all round the edge 

 of this blastoderm, cells are growing over the yolk and tucking- 

 in underneath the upper layer of the blastoderm to form endo- 

 derm. In fact, the edge of the blastoderm is the rim of the 

 blastopore, and mesoderm-cells are also proliferated from it. 

 The embryo forms in front of the posterior edge of the blasto- 

 derm, which is the dorsal lip of the blastopore, and does not 



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