THE DEVELOPMENT OF ORGANISMS 



m 



What the Germinal Layers Form. — The diversity among 

 animals is great and the detailed differences in their individual 

 development are very striking, yet it is noteworthy that there is a 

 deep similarity in the outcome of the three germinal layers. They 

 have a character or stamp, primarily dependent on their relative 

 positions, and in widely separated types they form the same struc- 

 tures. For backboned animals the embryological origins may be 

 summed up: 



{a) The ectoderm gives rise to the epidermis or outer skin and its 

 outgrowths, such as hair and feathers; the whole of the nervous 



Fig, 119. 



Early Development of Amphioxus. After Hatschek. A Primitive Vertebrate. 

 The blastula (A) becomes invaginated to form the gastrula (B). The 

 gastrula shows further differentiation, such as the formation of a dorso- 

 median neural plate (C, D). In E and F a suggestion of the origin of 

 the mesodermic pouches from the primitive gut or archenteron, of the 

 origin of the neural canal, and of the notochord or primary axis being 

 folded off from the roof of the archenteron. 



system; the most essential parts of the sense-organs, such as the 

 retina and the lens; an infolding or "fore-gut" at the mouth and a 

 trace of a "hind-gut" at the posterior end. 



{b) The endoderm gives rise to the mid-gut, which in Vertebrates 

 is practically the whole food-canal; the foundations of the many 

 pouches or diverticula that grow out from the gut, such as liver 

 and pancreas; and, somewhat surprisingly, an axial skeletal rod, in 

 most cases the predecessor of the mesodermic backbone. 



(c) It is convenient to take ectoderm and endoderm first, for then 

 one can say that the mesoderm forms all the rest, e.g. the under- 

 skin or dermis; the muscles : the connective tissue ; the bonj^ skeleton ; 



VOL. II B 



