266 OUTLINES OF CHORDATE DEVELOPMENT 



The intermediate cell mass in part forms the rudiment of the 

 excretory system, and in part contributes to the formation of 

 mesenchyme: its history will be described in connection with 

 the later stages of development. 



The lateral plate very early becomes separated into two dis- 

 tinct sheets by the development of a wide cavity within it. 

 This cavity is the ccelom, which is here, as in the frog, to be 

 described as a schizoccel. The outer or upper sheet of lateral 



op Ves 



n.f. 



F.G. 



<*././>. 



B 



FIG. 103. Ventral views of the head ends of chick embryos. From Lillie 

 (Development of the Chick). A. Embryo with five pairs of somites (about 

 twenty-three hours). B. Embryo with seven pairs of somites (about twenty- 

 five hours), a. c.i'., Amnio-cardiac vesicle; a.i.p., anterior intestinal portal; 

 End'c.s., endocardial septum; F.G., fore-gut; Ht., heart; My'C., myocardium ; 

 N'ch., notochord; N'ch.T., anterior tip of notochord; n.F., neural fold; op. Ves., 

 optic vesicle; p.C., parietal cavity (ccelomic) ; Pr'a., proamnion; s.2, s.4, second 

 and fourth mesodermal somites; V.o.m., omphalo-mesenteric vein. 



plate cells is the somatic mesoderm; this unites with the overly- 

 ing ectoderm to form the somatopleure (Figs. 102, 105, 108). 

 The inner or lower sheet is the splanchnic mesoderm; this 

 unites with the underlying endoderm to form the splanchno- 

 pleure. The somatic and splanchnic layers of mesoderm 

 remain united proximally, in the region of the intermediate 



