12 



MORPHOGENESIS 



The topography of the germinal areas in the embryonic disc shown in fig. 12 is based partly 

 upon a study of the succeedmg stages of development, and partly upon the results of experi- 

 ments upon the germinal disc in lower forms, especially in the chick (Assheton, Peebles, Kopsch). 



Law of developmental direction. — In the relative size of the various embryonic 

 areas is foreshadowed what maj^ be termed the law of direction in development. 

 In general it is found that development (including growth and differentiation) in 



Fig. 13. — Human Embryo 1.54 mm. long. Viewed from above, the roof of the amniotic 

 cavity having been removed. (Minot, after Graf Spee.) 



Yolk-sac 



Amnion 



Neural groove 



Neurenteric canal 



Primitive groove 

 Body-stalk 



Chorion with villi 



the long axis of the body appears first in the head region and progresses toward 

 the tail region. Similarly in the transverse plane development begins in the 

 mid-dorsal region and progresses latero-ventrally (in the limbs, proximo-distally). 

 These principles are of great importance in morphogenesis. 



Fig. 14. — Diagrams Showing the Constriction of the Embryo from the Yolk-sac. 

 A and C, longitudinal sections; B and D, corresponding cross-sections. (McMurrich.) 



The law of developnicritMl direction is also probably of jjhylogenetic significance. The 

 cranin-caudnl direction of development is in accordanc'e with tlie theory that the head is the most 

 primitive portion of the body, and hence precocious in development. The trunk is perhaps a 

 secondary acquisition, hence arising as an extension of the primitive head region. 



