EMBEYOLOGY. 



CHAPTEK I. 



CEPHALOCHORDA. 



The developmental history of the Chordata has been studied far 

 more completely than that of any of the groups so far considered ; 

 and the results which have been arrived at are of striking interest 

 and importance. Three main subdivisions of this group can be re- 

 cognized : (1) the Cephalochorda containing the single genus Am- 

 phioxus ; (2) the Urochorda or Tunicata ; and (3) the Vertebrata\ 

 The members of the second and probably of the first of these groups 

 have undergone degeneration, but at the same time the members 

 of the first group especially undergo a less modified development 

 than that of other Chordata. 



Cephalochorda. 



Our knowledge of the development of Amphioxus is mainly due 

 to Kowalevsky (Nos. i and 2). The ripe eggs appear to be dehisced 

 into the branchial or atrial cavity, and to be transported thence 

 through the branchial clefts into the pharynx, and so through the 

 mouth to the exterior. (Kowalevsky, No. i, and Marshall, No. 50 



When laid the egg is about 0105 mm, in diameter. It is in- 

 vested by a delicate membrane, and is somewhat opaque owing to the 

 presence of yolk granules, which are however uniformly distributed 

 through it, and proportionately less numerous than in the ova of most 



1 The term Vertebrata is often used to include the Cephalochorda. It is in many 

 ways convenient to restrict its use to the forms which have at any rate some in- 

 dications of vertebrae ; a restriction which has the further convenience of restoring to 

 the term its original limitations. In the first volume of this work the term Craniata 

 was used for the forms which I now propose to call Vertebrata. 



B. E. II. 1 



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