B 



PLATE 42. A section of a gastrula embryo of a frog. bp. Blastopore. From Marshall's 

 Vertebrate Embryology, by the courtesy of Smith, Elder and Co. B. A diagrammatic longitudinal 

 section of an older embryo of a frog. b. Blastopore. e. A layer of cells which will become 

 the lining of the alimentary canal. 11. A rod of cells (the notochord) which later is replaced 

 by the vertebral column. /. The so-called primitive streak where the notochord and the lining 

 of the alimentary canal fuse with the outer layer of the embryo, forming a plug of cells through 

 which opens the blastopore. The thickened part of the outer layer of the embryo, on the upper 

 side, will form the brain and spinal cord. C. A diagrammatic longitudinal section of the upper 

 portion of an embryo of a bird. Reference letters as in Fig. B, with which this figure should be 

 compared. The blastopore is very imperfectly developed and does not open. It is indicated only 

 by a thin spot in the primitive streak, which soon disappears. 



