i34 Vertebrate Embryology 



each side of the body becomes split by a series 

 of vertical clefts, at right angles to the long 

 axis of the body, and extending for a short 

 distance outwards from the notochord. Im- 

 mediately after the formation of these short, 

 transverse clefts, there is formed a vertical, 

 longitudinal cleft on each side of the noto- 

 chord, which divides each sheet of mesoblast 

 into a vertebral plate, lying next to the noto- 

 chord, and a lateral plate, lying further from 

 the median line. 



The lateral plate consists of two continuous 

 sheets of mesoblast, the somatopleure and 

 splanchnopleure ; while the vertebral plate is 

 divided by the transverse clefts into a series 

 of more or less cubical blocks, the mesoblastic 

 somites or proto-vertebrte (Figs. 44, p v, and 52, 

 Seg.). The first pair of somites is formed in the 

 neck region, and by the end of the first day, 

 there are usually five or six pairs present. The 

 second and possibly the third pairs of somites 

 are formed in front of the first pair ; and the 

 others, which appear in rapid succession, are 

 formed in regular order behind these first two 

 or three pairs. 



The number of pairs of somites at any given 

 time affords a convenient method of estima- 



