244 THE chick. 



iiotochord ; these are quickly followed by a pair of similar bul 

 longitudinal clefts, which appear one along each side of 

 body, a little distance from the middle line. By these clefts 

 the mesoblast of each side of the body becomes divided into a 

 vertebral plate, alongside the notochord ; and a lateral plate, 

 more peripherally placed ; the vertebral plate being further cut 

 up by the transverse clefts into a series of somewhat cubical | 

 blocks, the mesoblastic somites or proto-vertebrse (Fig. 110, ms). 

 The mesoblastic somites appear first in the neck region, and 

 increase rapidly in number during the last two hours of the 

 first day, and the following two or three days. One or perhaps 

 two pairs are formed in front of the pair which appears first ; the 

 remainder are added on in succession at the hinder end of the 

 series, as the embryo increases in length. At the twenty-fourth 

 hour of incubation there are usually five or six pairs present 

 (Fig. 110, MS); by the thirty-sixth hour (Fig. Ill) these have 

 increased to about fifteen pairs ; at the end of the second day 

 there are twenty-seven or twenty-eight pairs, after which date 

 the further increase takes place more slowly until, during the 

 fourth day, the full number is established. The increase takes 

 place in a very regular manner, and the number of somites 

 present affords a convenient basis for estimating the age, and the 

 grade of development, of embryos during the earlier stages of 

 their formation. 



The somites extend along the whole length of the neck, trunk, 

 and tail, but are not formed in the head, in which no segmenta- 

 tion of the mesoblast occurs. In the case of the first three or four 

 somites, the splitting of the mesoblast extends up to the notochord 

 before the somites become marked off from the lateral plates ; and 

 consequently the cavities of these somites communicate for a time 

 with the ccelom or cavity of the lateral plate, though this com- 

 munication is lost as soon as the longitudinal cleft is formed which 

 separates the vertebral and lateral plates from each other. 



The remaining somites, behind the first three or four pairs, 

 do not communicate at any stage with the ccelom, their cavities 

 appearing independently, and after the separation of the vertebral 

 from the lateral plates. 



The further stages in the development of the mesoblastic so- 

 mites will be described in a later section (p. 322). 



