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 coelom, which is here, as in the frog, to be 

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



op. Vea 



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.v., 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 



