III.] SUMMARY. 57 



(3) The primitive groove is formed along the centre of 

 the primitive streak. 



(4) The pellucid area becomes pear-shaped, the broad 

 end corresponding with the future head of the embryo. Its 

 long axis lies at right angles to the long axis of the egg. 



(5) The medullary groove makes its appearance in front 

 of the primitive groove, and below it the notochord is formed 

 out of mesoblastic cells. 



(6) The development of the head-fold gives rise to the 

 first definite appearance of the head. 



(7) The medullary folds rise up and coalesce in the 

 region of the neck to form the neural tube, the primitive 

 streak and groove disappearing. 



(8) One or more pair of protovertebrce make their ap- 

 pearance. 



(9) By the cleavage of the mesoblast, the somatopleure 

 separates from the splanclinopleure. 



(10) The first trace of the amnion appears in front of 

 the head-fold. 



(11) The vascular area begins to be be distinguished 

 from the rest of the opaque area. 



It may be well to remark, before passing on to the second 

 day, that out of the protovertebraa are formed not only the 

 permanent vertebrae, but also the superficial dorsal as well 

 as certain other muscles and the spinal nerves; that the pair 

 of protovertebrae first formed corresponds not with the first 

 cervical vertebra of the adult chick, but rather with the third 

 or even fourth ; for though the majority of the protovertebrce 

 are formed regularly behind the first pair, two or even three 

 pair may make their appearance in front of it; and lastly, 

 that in the part of embryo which forms the head, the meso- 

 blast is never cut up into protovertebrae, and never under- 

 goes cleavage to form somatopleure and splanchnopleure. 



