The Nervous System of Ambly stoma. 427 



the anus appears later as a new structure at the point where the 

 posterior end of the blastopore disappeared, a condition which 

 had already been described by Robinson and Assheton as "a 

 reopening of a temporarily closed blastopore." Morgan was 

 the first to observe that the thickened blastoporic lip was bounded 

 by a definite groove, which he saw, however, only in front of 

 the blastopore as a ''sickle-shaped" depression. This thickened 

 blastoporic lip is, according to Semon ('01), more prominent 

 in Ceratodus where it forms a broad circular rim surrounding 

 the slit-like blastopore, and this rim is bounded on the outer 

 edge by a circular groove, a condition which, as will be shown, 

 is very similar to that found in Amblystoma. 



The closing of the blastopore leaves on the surface of the egg 

 a narrow groove commonly called the primitive groove. This 

 structure was confused by the earlier investigators with other 

 grooves lying in front of it. Miss Johnson ('84) has recorded 

 her observation of but one groove running over the surface 

 of the egg for a distance equal to about three fourths of the 

 length of the neural plate. She homologizes the groove with 

 the primitive groove of higher vertebrates. Miss Johnson makes 

 one statement which rather refutes her theory of the presence 

 of but one groove when she says that " the front end of the primi- 

 tive groove deepens into a distinct pit." It will be shown that 

 in Amblystoma this pit is in reality a distinct groove which has 

 an entirely different origin from the groove which arises from 

 the closing of the blastopore. 



Robinson and Assheton ('91) consider only the posterior por- 

 tion of the long groove the true primitive groove, limiting the 

 term to that portion of the groove which is formed by the lips 

 of the blastopore. Jordan ('93) is more exact in his definitions. 

 He holds that the first or primitive groove never quite equals 

 the diameter of the original blastopore, and that it shows a 

 fusion of the primary germ layers throughout its length, while 

 the second groove shows no fusion of layers, although there is 

 an ''apparent fusion" at the anterior end where Miss Johnson 

 found an anterior pit. 



