94 THE DEVELOPMENT OF THE CHICK 
of very flat cells. The lateral extensions may be regarded as 
diverticula; subsequently these grow more rapidly at four places 
along their length, and come in contact with the ectoderm. Thus 
four pouches are established on each side as described in detail 
sis ee ee 
/ 
/ } - \ 
ne a 
ees 
Via. 48. — Stage of first intersomitie groove 
drawn from an entire mount in balsam by 
transmitted light. 
a.c. v., Amnio-eardiae vesicle. a. o., In- 
ner margin of Area opaca. Ect., Eetoderm. 
Ent., Entoderm  H. F., Head-fold.  i.s.f.1., 
First intersomitic furrow. med. pl., Medullary 
plate. Mes., Mesoderm. n. gr., Neural groove. 
pr. gr., Primitive groove. Pr’a, proamnion. 
in the next chapter. At the 12s stage one such place of contact 
is already formed, lying a short distance in front of the thickened 
ectoderm destined to form the auditory pit. 
