44 



AN INTRODUCTION TO ZOOLOGY 



forwards and inwards to leave the proamniotic area which is meso- 

 derm free. As the head fold grows forwards a narrow band of the 

 mesoderm situated along each side of the streak and notochord 

 becomes thicker and more opaque than the remaining parts. This 

 band is distinguished as the vertebral plate, paraxial or segmental 



zone, and the peripheral 

 thinner sheet is termed 

 the lateral plate or 

 parietal zone. It has 



..-w *'*" *-^r \ been P inted out also 

 / K : |W If ) that the lateral sheet of 



mesoderm has already 

 split to produce the 

 coelom which is therefore 

 a schizocoel as in the 

 frog. In the region just 

 behind the head process 

 the paraxial zone thins 

 out and loses its definite- 

 ness, and certain of its 

 cells migrate forward to 

 form a loose network of 

 cells in the fold itself, 

 thus constituting the 

 mesenchyme of the head, 

 to which additions are 

 made directly from the 

 ectoderm of the head 

 region. The great im- 

 portance of the differen- 

 tiation of the mesoderm 

 is that the somites are 

 derived from the paraxial 

 zone, while the parietal 

 zone is distributed to the 

 rest of the embryo. 



About twenty hours 

 after the beginning of 

 incubation a pair of 

 transverse furrows appear in the paraxial zone, a short distance in 

 front of the primitive knot and the mesoderm immediately anterior 

 to the furrow aggregates somewhat and forms an indistinct mass 

 that can be considered as the first somite, although it is never clearly 

 marked off from the rest of the front end of the zone. Shortly 



Fie. 140. Chick, twenty-four hours. From 

 Lillie. 



a.c.v., amnio-cardiac vesicle ; a.o., inner margin of area 

 opaca ; ect., ectoderm ; ent., entoderm ; A./., head 

 fold ; i.s.f.l., first intersomitic furrow ; med.pl., medullary 

 plate ; mes., mesoderm ; n.gr ., neural groove ; pr.gr., 

 primitive groove ; pr'a., proamnion. 



