Primitive Streak and Dorsal NotocJiordal Opening. 35 



that, after the entodermic streak has been overgrown by ectoderm, 

 and the primitive groove is formed, this primitive groove at its 

 front end sinks down into Kupffer's duct. According to Will, not 

 only a portion of the entodermic plug comes to lie on the ventral 

 floor of the primitive gut, but still later a portion of the streak 

 likewise occupies the same position. 



Robinson and Assheton ('91) had already criticised the com- 

 parison of the streak with the yolk plug as follows : " According 

 to Mitsukuri and Ishikawa, the streak consists in the first instance 

 mainly of a mass of hypoblast or yolk, which they compare to the 

 yolk plug of Amphibians. To this, however, it cannot correspond, 

 for we have already shown that the yolk plug of Rana is a portion 

 of the ventral wall of the archenteron, whilst the primitive streak 

 is formed by the fusion of the lateral lips of a deficiency in the 

 posterior wall of the same cavity." If Will had recognized the fact 

 that the lengthening of the notochordal canal is produced, in part 

 at least, by a progression backward of its dorsal opening, I believe 

 he would not have been drawn into these statements. He would 

 then have appreciated the fact that, during those stages in which 

 the neurenteric canal descends so obliquely anteriad, the anterior 

 or dorsal lips are fusing more rapidly than the posterior or ventral 

 lips are reopening. The apparent occupation of the ventral floor 

 by the primitive streak is only transitory. What now lies ventral 

 will divide, pass laterad, upward, and then mesiad to again fuse in 

 the dorsal lips in a position posterior to that which it at one time 

 occupied on the ventral floor. In later stages the opening of the 

 posterior lip more nearly keeps pace with the closure of the 

 anterior lip, so that the neurenteric canal comes to occupy a per- 

 pendicular position in relation to the embryo and yolk. I shall 

 recur to this point again, when my meaning will be made clearer 

 by the aid of diagrams. 



Will maintains that the primitive groove is formed upon the 

 entodermic streak by the growing over and union of the ectodermal 

 lips of the blastopore. It is formed, therefore, after the horns of 

 the blastopore bend posteriad. Mitsukuri ('93, pp. 259, 260) is 

 inclined to the same belief, although in an earlier paper he refers 

 to a primitive streak which exists posterior to and contemporane- 

 ous with the yolk plug. My sections, on the other hand, compel 

 me to conclude that during these later stages the primitive groove 

 disappears. It is at the time when the horns of the open blasto- 



