182 



POULTRY PRODUCTION 



of the egg, nornial (Icvelopmciit only takes place at nornial 

 temperatures, rcnningtoji and Pieree^ note that at 86° V. 

 to 91° F. seven or eight days arc re(|uired to equal three 

 days at the normal heat of incubation. The reverse, ho\ve\-er, 

 is true also. If the tem])crature is somewhat aljo\'e 103° V., 

 germination ])roceeds more ra])idly For instance, twenty- 

 four hours at 104° V to 100° V. gives a chick which is equal 

 in de\eloi)n^cnt to one incuhatcd for tlirce days at 10o° F. 

 In this connection the ol)scr\'ations <if .\ls(.ip on se\cnty-two- 

 liour embryos dcNcloiieil at high and low tem])cratures are 

 of great interest. (See page 208.) 



.ApIK-araru'C cf a rhicl^ eiiilir>'i) after t\^"eiif >--1miii iicui^ m :mi incul.atr.r. 

 (( niu ti'...y nl J\aii^a.-- Aili iciih aral I'^X] hi iniiail Slallon ) 



As soon after laying as the egg is placed at a normal incu- 

 bating temperature, cell division proceeds rapidly, and the 

 cells form thcmsel\-es into three layers which, from without 

 to witliin, are referred to as tlie ci'todcrm, mesoderm, and 

 endoderm. These three layers are the beginnings of three 

 distinct (li\isions of tlie body. 



From the ectoderm, the skin, feathers, beak, claws, ner- 

 vous system, lens, and retina of the eye and linings of the 



'Year-book. United States Department of Agricullnrc, 1910. 



