BASIC HOMOLOGY OF ORGAN SYSTEMS 



547 



may be deleted along the respective tubes. Regardless of this possibility, a 

 basic homology, however, appears to exist. 



During later development through larval and definitive body-form stages, 

 a considerable amount of molding or plasis by environmental and intrinsic 

 factors may occur. An example of plasis is given in the development of the 

 forelimb rudiment of the fish, frog, bird, and pig. In the definitive form, these 

 structures assume different appearances and are adapted for different func- 



METENCEPHALON 



BASILAR ARTERY 

 NOTOCHORD 



ROOT OF TON GUE 

 THYROID GLAND 

 DEVELOPING EPIGLO 

 AORTIC ARCH 



TRAC 

 ESOPHAG 



SPINAL CORD 



MESENCEPHALON 



TUBERCULUM 

 POSTERIUS 

 INFUNDIBULUM 

 DIENCEPHALON 

 RATHKE'S POCKET 

 EESSEL'S POCKET 



PTIC CHIASMA 



CESSUS OPTICUS 



LENCEPHALON 

 INA TERMINALIS 



IINUS VENOSUSp^j 

 DORSAL -l--_i_ 



PANCREAS 

 DUODENUM 



GALL BLADDER 



NOTOCHORD 



JORSAL AOR 



MESONEPHR 

 KIDNEY 



SEPTUM TRANSVERSUM 

 DUCTUS VENOSUS 



EXTRA-EMBRYONIC COELOM 



UMBILICAL CORD 



ALLANTOIC DIVERTICULUM 

 ' ' ^> GENITAL EMINENCE 

 PROCTODAEUM 



ALLANTOIC STALK 



METANEPHRIC 



METANEPHROGENOUS 



TISSUE SPINAL GANGLION 



Fig. 261 — (Continued) (B) Median sagittal section of 10 mm. embryo. 



