Development of the Fourth Day 227 



place to describe the entire development of 

 that important structure in more or less detail. 



During the fourth day, the mesoblastic 

 somites increase in number from about thirty 

 to forty, and each somite has been divided, as 

 has been explained, into an outer part, or 

 muscle plate, and an inner part of less differ- 

 entiated mesoblast from which the vertebral 

 column will be developed. This mesoblast 

 increases in amount, and by sending out pro- 

 cesses both above and below the neural canal, 

 and also below the notochord, these structures 

 become completely surrounded by mesoblast, 

 which, by the end of the fourth day, has 

 acquired a considerable thickness, and is some- 

 times known as the membranous vertebral 

 column. This membranous vertebral column 

 still retains the tranverse lines of segmentation 

 of the original mesoblastic somites. Early on 

 the fifth day, however, these lines of demarca- 

 tion disappear, and the mesoblast surrounding 

 the neural canal and the notochord becomes 

 a continuous tube. This does not apply to 

 the muscle plates, which retain their original 

 planes of segmentation. 



During the fifth day, the mesoblast in im- 

 mediate contact with the notochord becomes 



