PRIMITIVE GROOVE IN THE EMBRYO CHICK. 45 



the bottom of the now much expanded medullary groove has 

 become raised to form the ridge which separates the medullary 

 from the primitive groove. The medullary folds are also flatter 

 and broader than in the previous section. Section 4 (fig. 16) 

 passes through the anterior end of the primitive groove. Here 

 the notochord is no longer visible, and the adherence between 

 the mesoblast and epiblast below the primitive groove comes 

 out in marked contrast with the entire separation of the two 

 layers in the previous sections. 



The medullary folds (A) are still visible outside the raised 

 edges of the primitive groove, and are as distinctly as possible 

 separate and independent formations, having no connection with 

 the folds of the primitive groove. In the last section (fig. 17), 

 which is taken some way behind section 4, no trace of the 

 medullary folds is any longer to be seen, and the primitive 

 groove has become deeper. This series of sections, taken in 

 conjunction with the specimen figured in fig. 1 8, must remove all 

 possible doubt as to the total and entire independence of the 

 primitive and medullary grooves. They arise in different parts 

 of the blastoderm ; the one reaches its maximum growth before 

 the other has commenced to be formed ; and finally, they are 

 distinguished by almost every possible feature by which two 

 such grooves could be distinguished. 



Soon after the formation of the notochord, the proto-vertebrse 

 begin to be formed along the sides of the medullary groove (PI. 

 I, fig. 19, pv). Each new proto-vertebra (of those which are 

 formed from before backwards) arises just in front of the an- 

 terior end of the primitive groove. As growth continues, the 

 primitive groove becomes pushed further a"nd further back, and 

 becomes less and less conspicuous, till at about thirty-six hours 

 only a very small and curved remnant is to be seen behind the 

 sinus rhomboidalis ; but even up to the forty-ninth Dursy has 

 been able to distinguish it at the hinder end of the embryo. 



The primitive groove in the chick is, then, a structure which 

 appears very early, and soon disappears without entering di- 

 rectly into the formation of any part of the future animal, and 

 without, so far as I can see, any function whatever. It is clear, 

 therefore, that the primitive groove must be the rudiment of 

 some ancestral feature ; but whether it is a rudiment of some 



