500 



DEVELOPMENT OF PRIMITIVE BODY FORM 



ANTERIOR NEUROPORE MANDIBULAR CONTRIBUTION TO FIRST BRANCHIAL GROOVE 



^"■''E*"^*'- EAR (EXTERNAL AUDITORY MEATUS) 



HYOI D 

 TRIBUTIONS TO 

 EXTERNAL EAR 



DIFFE RENTI ATING 

 SOMITES 



Fig. 246. Development of body form in human embryo. (C from Keibel and Mall: 

 Manual of Human Embryology, Vol. I, 1910. Philadelphia and London, Lippincott. 

 A, B, D, and E from Keibel and Elze: Normentafel zur Entwicklungsgeschkhte des 

 Menschen. Jena, 1908. G. Fischer.) (A) Early neural fold stage. Somites are beginning 

 to form; notochordal canal is evident. (B) About nine pairs of somites. (C) His's 

 embryo M. (D) About 23 pairs of somites, 4-5 mm. long. (E) About 35 pairs of 

 trunk somites, 12 mm. long. 



mesodermal, and entodermal tubes and with them it continues to grow in 

 length principally by rapid cell proliferation at the caudal end of the embryo. 



4. TUBULATION OF THE EnTODERMAL ArEA 



The primitive metenteron of Amphioxus is derived from the archenteron 

 of the late gastrula as follows. 



a. Segregation of the Entoderm from the Chordamesoderm and the 

 Formation of the Primitive Metenteric Tube 



The mesoderm and notochord which occupy the roof of the archenteron 

 of the gastrula evaginate dorsally at the anterior end of the embryo and, thus, 

 become separated from the entoderm. (Compare fig. 195 with fig. 250A.) 

 This separation of notochord and mesoderm by dorsal evagination from the 



