FACTS AND FACTORS OF DEVELOPMENT 37 



are the same in oviparous and in viviparous 

 animals — in fishes, frogs, birds and men. 



Summary. — This is a very brief and incom- 

 plete statement of some of the important stages 

 or phases of the development of the body of 

 man or of any other vertebrate. In all cases 

 development begins with the fertilized egg 

 which contains none of the structures of the de- 

 veloped animal, though it may exhibit the 

 polarity and symmetry of the adult and may 

 also contain specific kinds of protoplasm which 

 will give rise to specific tissues or organs of 

 the adult. From this egg cell arise by divi- 

 sion many cells which differ from one another 

 more and more as development proceeds, until 

 finally the adult animal results. A specific 

 type of development is due to a specific organ- 

 ization of the germ cells with which develop- 

 ment begins, but the earlier differentiations 



are closing and five pairs of somites have appeared; age, ten 

 to fourteen days. D, stage of fourteen somites showing en- 

 largements of the neural folds at the anterior end which will 

 form the brain; age, fourteen to sixteen days. E and F later 

 stages, the latter with twenty-three somites and three visceral 

 clefts. The ear shows as a depression at the dorsal angle of 

 the second cleft. O, embryo of thirty-five somites showing eye, 

 branchial arches and limb buds. H, embryo of thirty-six 

 somites showing nasal pit, eye, branchial arches and clefts, 

 limb buds and heart. (After Keibel.) 



