CHAt>. X.J THE DEVELOPMENT OF THE CAT. 541 



epiphysis is the first to appear and the last to unite. In the fibula 

 the superior epiphysis is both the last to appear and also to unite. 



The tarsals each ossify from one centre, as do the carpals, except 

 the OS calcis, which has also an epiphysis at the end of its tuberosity. 



The metatarsals and phalanges of the hind-foot (or pes) ossify in 

 the same way as do the corresponding parts of the fore-foot (or 

 manus), except that the hallux remains in a very rudimentary con- 

 dition. The bony parts of the hind-paw ossify at about the same 

 time as do those of the fore-paw. 



§ 12. The MUSCLES of the trunk arise in what are called the 

 "muscle-plates " of the primordial vertebrEe, forming, as such plates 

 do, the upper part of each dorsal segment. Hence are derived the 

 erector spinas mass of muscles, the segments of which primitively 

 correspond in number and position with the vertebrae themselves. 

 The muscles of the abdominal wall are formed in the mesoblast of 

 the somatopleure. The sub-vertebral muscles, such as the longus 

 coUi, &c., are formed in the mesoblast, which lies nearer the centre 

 than the division between the somatopleure and splanchnopleure 

 ever extends. The muscles of the limbs do not arise as protrusions 

 of the body muscles outwards. They are directly formed in 

 the mesoblast of the budding members themselves. 



§ 13. The ALIMENTARY CANAL is formed, as has been already 

 intimated, by the splanchnopleure and hypoblast. It is at first in 

 the form of an axially directed groove placed beneath the notochord, 

 and opens downwards towards the yelk — and therefore is turned in 

 the opposite direction to the medullary groove above it. The walls 

 of the groove are, on each side and in front and behind, formed by 

 mesoblast, everywhere lined by the hypoblast. This groove soon 

 changes (by the descent and convergence of the splanchnopleure on 

 both sides of it) into a canal. At each end {i.e., at the head and 

 tail end) of this canal, the descending ventral lamina) (which are there 

 undifierentiated into somatopleure and splanchnopleure), close it in; 

 so that, as has been already said, the incipient alimentary canal at 

 first ends blindly both in front and behind, though in its middle it 

 remains open, communicating with the yelk or vitelline sac by 

 means of the vitelline duct. When the communication is almost 

 closed up, the vitelline sac takes the name of the umbilical vesicle, 

 and on it ramify certain blood-vessels, which are, for a time, of 

 great importance. 



The incipient alimentary canal is straight, cylindrical and attached 

 to the underside of the vertebral column by a wide but thin layer of 

 mesoblast. After a time the tube, towards the middle of the body, 

 bends down away from the vertebral column, pulling out, as it were, 

 the interposed layer of mesoblast into a thin vertically extended 

 membrane, the future mesentery. Part of the tube on the anterior 

 (cephalic) side of this loop or curve, dilates more and more and 

 becomes the stomach ; the part on the posterior side of the loop be- 

 comes the transverse colon. To the most prominent and middle part 

 of the loop itself, the pedicle of the umbilical vesicle is attached. In 



