238 EARLY STAGES OF ARTHROPOD AND VERTEBRATE EMBRYOS. 



IV. THE CEPHALIC NAVEL, DORSAL ORGAN, OR NEOSTOMA. 



Linmlus. During the primitive cumulus stage, the blastoderm outside the 

 germinal area consists of a single layer of sharply defined cells. At this period, 



.-.' .'.\: : .. : --\;.; t.',".;. i;^/!6.<"* ; V" -V/**? 



F %jp 



FIG. 140. Limulus embryos in stage F and stage G, the latter in mercator projection. The lateral plates of the 

 sixth thoracic and anterior abdominal metameres, and the segmental sense organs, are clearly shown. 



^:.-:^m^^^^m^^ : -:. 



. ^W&iMtmmi&s-: ; mms^.K^y:-. - 



FIG. 141 . Same, stage H, mercator projection. The first five thoracic metameres and the procephalic lobes have no 



segmented lateral plates. 



the "vicarious chorion," a thick membrane secreted by the superficial cells of the 

 eggs, is smooth and structureless over the germinal area, but over the blastoderm 

 it is divided into polygonal plates, one for each blastoderm cell. (Fig. 128, v.c.) 



