380 C. 0. WHIT.MAN\ 



the trunk of the embryo, while Dursy and Balfour assert that 

 it entirely atrophies, or at most (Balfour) forms only the " tail 

 swelling ^^ and a part of the ventral wall of the post-anal gut; 

 and when His and Rauber maintain that the embryo is formed 

 by a retrogressive concrescence of the two symmetrically placed 

 halves of the germinal ring, while Balfour and others contend 

 that there is no such concrescence, and that the lengthening of 

 the embryo is entirely by intussusceptional growth, it might 

 seem like presumption on ray part to offer any suggestions on 

 these disputed points. It is far from my intention, however, 

 to enter into an elaborate consideration of all these questions, 

 and I have no conclusions to present which will take me very 

 far from the plain highway of observed fact. 



As to the meaning of the primitive groove, I think the 

 blastoderm I have described furnishes a very strong confirma- 

 tion — not to say verification — of a theory that originated with 

 Balfour and Rauber ; namely, that this groove represents 

 a portion of the blastopore. It was this theory that con- 

 ducted Rauber to a correct interpretation of the marginal 

 notch. No one had ever discovered any connection between 

 the notch and the primitive groove, or suggested any such 

 relation between the two structures. The notch was of rather 

 rare occurrence, and therefore treated as an irregularity that 

 required no explanation. Its position directly behind the pri- 

 mitive groove appeared to Rauber to fit in with the opinion 

 that this groove extended originally to the very edge of the 

 blastoderm, precisely as it still does in Elasmobranch Fishes ; 

 and hence he regarded it as " the ideal hind end of the groove." 

 This conjecture is raised to the dignity of an observed fact in 

 the present case, and its verification is complete, provided the 

 blastoderm here considered represents an atavistic form. 

 Comparative embryology must be our guide in a question of 

 this nature. 



All the questions that cluster about the primitive streak are 

 only so many special sides of a more general question, namely, 

 How is the embryo formed? On the decision of this ques- 

 tion hangs that of all the rest. Two different views have 



