ANIMAL BIOLOGY. [Part I. 



form the epiblast. In the peripheral area around this the outer 

 cells form flattened epiblast, and the inner cells form hypoblast. 

 The lower part of the vesicle is constituted by a single layer of 

 outer cells forming flattened epiblast. In the course of develop- 

 ment the hypoblastic layer grows outwards, so as to line this 

 part also. So that we have a state of things somewhat analo- 

 gous to that which obtains in the fowl ; but with this difference, 

 that in the fowl the blastoderm grows round and encloses a mass 

 of yolk, while in the rabbit it grows round and encloses a cavity 

 containing an albuminous fluid. And just as in the fowl it is 

 only the pellucida area which gives rise to the embryo, so in the 

 rabbit it is only the embryonic area that gives rise to the 

 developing organism. 



In this area the primitive streak is formed in a manner very 

 similar to that which obtains in the fowl ; its surface becomes 

 furrowed with a primitive groove, and from it the mesoblast of 

 the posterior part of the embryonic area is produced (Fig. 35, 

 iv.). In the anterior part of this area, where the medullary 

 groove appears, the mesoblast is derived, as in the chick, from 

 the hypoblastic layer (Fig. 35, v.). From the hypoblast of the 

 axial line the notochord is developed ; and on either side are 

 formed the mesoblastic plates,, separating into (1) a vertebral 

 plate with its mesoblastic somites, in which the body-cavity, 

 arising, as in other types, by cleavage of the mesoblast, is 

 obliterated, and (2) lateral plates, in which the cleavage of the 

 mesoblast and resulting body-cavity is persistent. 



The Neural Tube. In all three types the mode of forma- 

 tion of the neural tube differs in no essential from that de- 

 scribed in our illustrative case. A longitudinal neural plate 

 is formed in the mid-dorsal line* On either side neural folds 

 (lamince dorsales) grow up, and, eventually meeting above, enclose 

 a canal. In the frog this takes place along the whole dorsal 

 line (Fig. 36, i.), and the canal opens into the blastopore. In 

 the fowl and rabbit it is formed in the anterior two-thirds of the 

 embryonic area, and the two neural folds diverge posteriorly, 

 so as to enclose the front end of the primitive streak 



