564 EMBRYOLOGY OF TADPOLE AND CHICK 



has been carried to the axial region. Invagination, in the frog, is there- 

 fore considered of minor importance. 



Amphioxus is the only chordate which remains in a two-layered 

 state for some time. All other chordates retain this condition for a short 

 period, and this is so because the mesoderm begins its development al- 

 most at the moment of gastrulation. 



In fact, in the frog, mesodermal cells are found almost immediately 

 after the entoderm cells begin, to form. They are found, first, all around 

 the margin of the blastopore where they form an important part of the 

 germ-ring. Mesoderm which forms in this way is called blastoporal or 

 peristomial. 



As confluence begins, the lateral portions of the germ-ring are 

 brought .closer to the mid-dorsal region where they become a part of 

 the axis of the elongating embryo. That is, they form the mesodermal 

 bands, already referred to, and it is these bands of the germ-ring which 

 become axial in position, then to be known as gastral mesoderm. This 

 gastral mesoderm is nothing more nor less than mesoderm derived in 

 turn from the blastoporal mesoderm. It is no different from any other 

 mesoderm derived from the same origin, although it lies, of course, in 

 a different position from the remaining mesoderm. In Amphioxus, the 

 above statement, however, is not true ; for, in that animal, gastral meso- 

 derm and blastoporal mesoderm have different origins. This difference 

 in origins may be traced to the fact that in the frog the mesoderm dif- 

 ferentiates before gastrulation and confluence, while in Amphioxus, the 

 mesoderm does not differentiate until after these two processes have 

 begun. 



THE FORMATION OF THE EMBRYO 



All that has been discussed so far has taken place within two days 

 after fertilization. Now the embryo can be seen either lying in a straight 

 line on the dorsal surface or slightly concave, while the ventral surface 

 appears' convex. The ectoderm still forms the entire covering epithelium, 

 although some of these ectodermal cells have developed cilia which 

 appear just before the fusion of the neural folds. These cilia beat in a 

 posterior direction, thus giving the embryo a slow, rotary motion within 

 the egg membranes. 



-The transverse neural fold marks the anterior limit of the nervous 

 system, while r ,,the posterior limit is located just anterior to the dorsal 

 lip of the blastopore;. 



As confluence continues, the nervous system comes to extend from 

 almost pole to pole on the posterior surface of the gastrula, even- before 

 the blastopore has entirely contracted. The -gastrula then rotates so 

 that this posterior portion becomes dorsal, thus bringing the transverse 

 neural fold to the anterior end of the nervous system, while the medul- 

 lary plate occupies nearly the entire dorsal surface of the embryo. 

 fj ..:,- The ridges which form on the neural plate and later fuse in a similar 



