STRUCTURE OF THE EMBRYO AT HATCHING 



SPINAL CORD 



NOTOCHORD 

 DORSAL AORTA 



NEPHROSTOME 

 PRONEPHRIC TUBULE 

 PRONEPHRIC DUCT 



137 



NEURAL CREST 



DERMATOME 



MYOTOME 



SCLEROTOME 



POSTERIOR 



CARDINAL 



VEIN 



GLOMUS 



SPLANCHNIC LAYER 

 OF MESODERM 



SOMATIC LAYER 

 OF MESODERM 



COELOM 



Fig. 79. A section through the somites and pronephros. The pronephros and 

 its relation to the blood system is highly diagrammatic. An actual cross section of 

 an embryo does not show all these relationships. Several cross sections must be 

 studied in order to reconstruct the essential parts of the excretory system. 



cloaca. The other end of the outgrowth opens into the coelom (nephrocoel) 

 by means of a nephrostome, which develops cilia. The cilia sweep excretions 

 from the coelomic cavity into the nephrostome and through the tubule into 

 the pronephric duct. The tubule becomes long and convoluted. In Figure 79 

 the structures are shown in diagrammatic form in order to emphasize func- 

 tional relationships. 



