No. 2.] EARLY DEVELOPMENT OF AMBLYSTOMA. 377 



3. The primitive streak represents the part of the blasto- 

 pore between the neurenteric canal and anus, Schwarz ('89). 



4. The primitive streak represents the entire fused area 

 around the rim of the blastopore, Robinson and Assheton ('9i). 



The term "primitive streak" as is well known, was first ap- 

 plied to an opaque streak in the posterior portion of the area 

 pellucida of the chick; this streak, according to Balfour, is due 

 to the presence of a third layer, the mesoblast. The later 

 authors have quite generally agreed that the primitive streak 

 is the portion of the blastoporic area where all three layers are 

 fused. 



This area of fusion, as stated in a preceding page, extends 

 around the entire blastoporic margin, so that in Amblystoma 

 the basis of the primitive streak is a well defined ring. It is 

 not, however, until the closure of the blastopore that the 

 structure is generally referred to as such. 



I wish here to refer to a structure which has been inter- 

 preted as a more or less exact criterion of the extent of the 

 primitive streak, viz., the " dorsal groove." It would indeed 

 be a hopeless task to attempt the enumeration of synonymous 

 terms which have been given by the various authors, or to 

 record the many views regarding its position and extension. 



In Rana temporaria, according to Hertwig ('82), the dorsal 

 groove extends forward from the blastopore, while in the same 

 form Robinson and Assheton ('91) find it extending backward 

 from the blastopore. In the Newt Hertwig ('82) never finds 

 the groove continuous with the edge of the blastopore, while 

 Miss Johnson ('84) often finds this occurring. Jordan ('93) ob- 

 served the primitive streak and neural grooves to later unite 

 and form one structure. In a recent publication Bambeke 

 ('93) has endeavored to show that the dorsal groove (Riicken- 

 rinne, sillon median) represents the primitive groove and is the 

 outward expression of the extent of the primitive streak. The 

 author says : 



" On peut admettre avec O. Hertwig, que le sillon median 

 ou dorsal (Ruckenrinne) des Amphibies repr^sente la ligne de 

 suture, suivant laquelle, en vertu de leur rapprochement lent 

 d'avant en arri^re, les l^vres du blastopore se sont juxtapos^es 



