EXISTING ORGANISMS— EMBRYOLOGY G7 



brief account the most conspicuous examples have been selected 

 and shorn of all unnecessary details. Such an account is neces- 

 sarily imperfect, but in this field even scattered facts are striking. 

 Summary. The relationship of vertebrates is strikingly evident 

 in their embryology. A study of the developmental stages of the 

 various classes shows that animals pass through similar steps up 

 to the point where the divergence of adult structure appears. 

 Even in the initial stages of cleavage and gastrulation modifica- 

 tions occur, but these are incidental to the storage of yolk in the 

 ovum and do not destroy the homologies. After gastrulation such 

 structures as the neurenteric canal and the foetal membranes show 

 other definite relationships. Not only do the embryos resemble 

 each other, but the embryos of higher classes also pass through 

 stages similar to the maximum development of classes below them. 

 The early stages of embryonic development are similar to some of 

 the invertebrates. Such structures as the notochord, the skeleton, 

 and the circulatory and respiratory systems show a gradual transi- 

 tion in adults from the cyclostomes to the mammals which is also 

 followed during ontogen3\ The entire development of the indi- 

 vidual is a story of relationships which are illustrated by these 

 selected examples. 



REFERExXCES 



Newman, H. H., Vertebrate Zoology, 1920. 



Wilder, H. H., History of the Human Body, revised edition, 1923. 

 McEwEN, R. S., A Textbook of Vertebrate Embryology, 1923. 

 Arey, L. B., Developmental Anatomy, 1924. 



