STRUCTURE OF TWENTY-FOUR HOUR CHICKS 49 



In the chick the lateral mesoderm Hke the intermediate 

 mesoderm, shows no segmental division. In 24-hour embryos 

 (Fig.. 17, C) it is clearly differentiated from the intermediate 

 mesoderm by being spHt horizontally into two layers with a 

 space between them. The layer of lateral mesoderm lying 

 next to the ectoderm is termed the somatic mesoderm, the layer 

 next to the entoderm is termed the splanchnic mesoderm and 

 the cavity between somatic and splanchnic mesoderm is the 

 ccelom. Because in development the somatic mesoderm, and 

 ectoderm are closely associated and undergo many foldings in 

 common, it is convenient to designate the two layers together 

 by the single term^^SQjnatopleure. Similarly the splanchnic 

 mesoderm and the entoderm together are designated as the 

 splanchnopleure. 



The Coelom. — The coelom, Hke the cell layers of the blasto- 

 derm extends over the yolk peripherally beyond the embryonal 

 area (Fig. 17, C). Later in development foldings mark off the 

 embryonic from the extra-embryonic portion of the germ layers. 

 This same folding process divides the coelom into intra-em- 

 bryonic and extra-embryonic regions. In the 24-hour chick, 

 however, embryonic and extra-embryonic coelom have not been 

 separated. 



It is evident from the manner in which the coelomic chambers 

 arise in the lateral mesoderm that the coelom of the embryo con- 

 sists of a pair of bilaterally symmetrical chambers. It is not 

 until later in development that the right and left coelomic 

 chambers become confluent ventrally to form an unpaired 

 body cavity such as is found in adult vertebrates. 



The Pericardial Region. — In the region of the anterior intes- 

 tinal portal the"' coeTomic chambers on either side show very 

 marked local enlargements. Later in development these 

 dilated regions are extended mesiad and break through into 

 each other ventral to the fore-gut to form the pericardial cavity. 

 In their early condition these enlarged regions of the coelomic 

 chambers are usually called amnio-cardiac vesicles. With their 

 later fate in mind we may avoid multiplication of terms and 

 speak of them from their first appearance as constituting the 

 pericardial region of the coelom. , 



The relationships of the pericardial region of the coelom in 

 embryos of 24 hours can be most readily grasped from a study 



