THE URINOGENITAL SYSTEM. 345 



flexure of the vertebral column are clearly that each muscle- 

 segment shall be capable of acting on two vertebrae; and this 

 condition can only be fulfilled when the muscle-segments are 

 opposite the intervals between the vertebrae. Owing to this 

 necessity, when the vertebral segments became formed, their 

 centres corresponded, not with the centres of the muscle-plates, 

 but with the inter-muscular septa. 



These considerations fully explain the secondary segmen- 

 tation of the vertebrae by which they become opposite the inter- 

 muscular septa. On the other hand, the primary segmentation 

 is clearly a remnant of the time when no vertebral bodies were 

 present, and has no greater morphological significance than the 

 fact that the cells to form the unsegmented investment of the 

 notochord were derived from the segmented muscle-plates, and 

 only secondarily became fused into a continuous tube. 



The Urinogenital System. 



The first traces of the urinary system become visible at 

 about the time of the appearance of the third visceral cleft. At 

 about this period the somatopleure and splanchnopleure become 

 more or less fused together at the level of the dorsal aorta, and 

 thus, as has been already mentioned, each of the original plates 

 of mesoblast becomes divided into a vertebral plate and lateral 

 plate (PI. 11, fig. 6). The mass of cells resulting from this fusion 

 corresponds with Waldeyer's intermediate cell-mass in the Fowl. 



At about the level of the fifth protovertebra the first trace of 

 the urinary system appears. 



From the intermediate cell-mass a solid knob grows outwards 

 towards the epiblast (woodcut, fig. 4, pd\ This knob consists at 

 first of 20 30 cells, which agree in character with the neigh- 

 bouring cells of the intermediate cell-mass, and are at this period 

 rounded. It is mainly, if not entirely, derived from the somatic 

 layer of the mesoblast. 



From this knob there grows backwards a solid rod of cells 

 which keeps in very close contact with the epiblast, and rapidly 

 diminishes in size towards its posterior extremity. Its hinder- 

 most part consists in section of at most one or two cells. It 

 keeps so close to the epiblast that it might be supposed to be 

 B. 23 



