EMBRYOLOGY. 



CHAPTER I. 

 CEPHALOCHORDA. 



THE developmental history of the Chordata has been studied 

 far more completely than that of any of the groups so far con- 

 sidered ; and the results which have been arrived at are of 

 striking interest and importance. Three main subdivisions of 

 this group can be recognized : (i) the Cephalochorda containing 

 the single genus Amphioxus ; (2) the Urochorda or Tunicata ; 

 and (3) the Vertebrata 1 . The members of the second and 

 probably of the first of these groups have undergone degenera- 

 tion, 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. 1 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. 1, and 

 Marshall, No. 5.) 



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 indica- 

 tions 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. III. I 



