116 OUTLINES OF CHORDATE DEVELOPMENT 



mobility of the blastomeres and the epithelia which they 

 compose. 



We may assume that the stage in which the rudiments of the 

 central nervous system, notochord, mesoderm, and gut are all 

 definitely established, marks the end of notogenesis and the 

 beginning of the formation of a definite embryo in a restricted 

 sense. 



B. THE FORMATION OF THE EARLY EMBRYO 



We may close our account of the early embryonic period by 

 tracing briefly the further development of the rudiments 

 formed at the close of notogenesis, up to the time the neural 

 tube is closed. We have assumed arbitrarily to let this stage 

 (about two days after fertilization) represent an "early embryo" 

 (Fig. 22, E, F}. At this time the_ejnbryo has elongated so that 

 its length is about one and one-half times its depth or the diam- 

 eter of the gastrula. The postero-dorsal region is narrowed and 

 drawn out slightly into the rudiment of the_tail. The dorsal 

 surface is straight or even slightly concave and narrowed in 

 cross section, the ventral surface remains broadly convex. The 

 rudiments of several organs are visible externally as elevations 

 or depressions; these will be described in connection with the 

 internal structure. Externally the ectoderm alone still forms 

 the covering epithelium, for as yet no part of the mesoderm has 

 contributed to the formation of an integument. About the only 

 change in the character of the ectoderm is the development, on 

 some of its cells, of numerous short cilia. Just before the fusion 

 of the neural folds the cilia begin to appear first along their 

 margins. They extend rapidly more widely over the surface, 

 and by the time this early embryonic stage is reached they are 

 absent from only the ventral surface. A little later the ciliation 

 is completed. The cilia beat in the posterior direction and give 

 the embryo a slow rotary motion within the egg membranes. 



1. The Nervous System 



It is more convenient to describe the development of the 

 nervous system first, partly because many of the chief external 



