LATER DEVELOPMENT OF CHICK AND RABBIT 423 



anastomose and branch freely, giving rise to a network of liver 

 tissue around the venous trunks. With the further growth of this 

 glandular structure its branches penetrate the ductus venosus, 

 pushing its wall in front of them, and finally they cut it up into a 

 canalisation of small vessels superficially resembling capillaries, but 

 termed sinusoids, to indicate their different origin. These are 

 retained as the venous vessels of the adult liver, and so the 

 ductus pours its biood into them instead of directly into the sinus 

 venosus, and forms the foundation of the definitive hepatic portal 

 system. 



The pancreas arises a little later than the liver, and by three 

 diverticula. The dorsal outgrowth appears on the dorsal gut wall 

 about opposite the posterior liver diverticulum during the third day. 

 The two ventral rudiments appear behind the latter, and soon after 

 the dorsal one. In a short time the three separate pancreatic growths 

 fuse to form a common mass. 



Later History of the Mesoderm, 



The formation of the mesoderm and its differentiation into 

 somites, intermediate cell mass and lateral plate have already been 

 treated, but certain points in connection with their later history call 

 for brief notice. By the fourth day 42 somites-have been laid down, 

 and after that 10 more are formed, but disappear later. The somites 

 are distributed in the following manner : 1-4 enter into the skull ; 

 5-16 between the skull and fore limb ; 17-19 fore limb ; 20-25 

 between fore and hind limb ; 26-32 hind limb ; 33-35 cloaca! region ; 

 and 36-42 caudal region. The transitory somites are situated 

 behind the last of these, and their disappearance suggests that an 

 ancestral form possessed a longer tail. 



At an early stage in each somite, as we have seen, an epithelial 

 and a more solid portion can be recognised, but later we find that 

 three parts become differentiated, viz. the dermatome, the myotome, 

 and the sclerotome. The original dorsal epithelial part becomes 

 more distinct, and its lateral region becomes fairly sharply delimited 

 from the remainder as the cutis plate or dermatome. The mesial 

 dorsal portion also becomes marked out as the muscle plate or 

 myotome. This becomes thin and turns in under the cutis plate. 

 It grows outwards, and finally reaches and fuses with the free lateral 

 edge of the cutis plate, so that the two form a double-layered plate, 

 sometimes referred to as the dermo-myotome. With the ingrowng 

 of the lateral limiting sulcus this plate becomes disposed practically 

 vertically. Its outer layer, the cutis plate, is gradually transformed 

 into the dermis of the adult, and as it does so it becomes more and 

 more closely attached to the ectoderm, which provides the epidermis. 



