104 



K. MITSCKURl 



(Neuretit C.) of the neurenteric canal* but behind if, the hps of the 

 blastopore pressing from both sides towards, and meeting in, the 

 median line have removed by their proliferation the remnant of the 

 primary yolk-mass (Ydi. 1) some distance behind the posterior end 

 of the embryo, producing the primitive streak. 



In IX, the blastoderm has covered the secondary yolk-mass 

 (Ym. 2) to the line e-c. In the region of the primitive streak, tlie 

 tail-swelling has arisen, changing the liitherto straight primitive streak 

 into a 8"shaped curve (IXa). The upper part of this figure corres- 

 ponds to the whole of the e(i:g in IV. Yni. 1 is in both cases the 

 yolk-blastopore. Between this (Yiii. 1) and the neurenteric canal 

 (Xeuroit. C.) there is in both figures a, 8-shaped streak. The simi- 

 larity is too striking to need any further explanation. Xote that the 

 final enclosure of the secondary yolk-mass takes place at the pole 

 opposite tliat which the blastoderm occupies in \, instead of being at 

 some spot oUhpie to the original axis as in the case of the primary 

 yolk-mass in IV. My views beautifully account for this difference. 



Thus far, the explanation of the diagrams. 



In that series of diaüTams mven bv Hektwig, the edge of the 

 blastoderm is divided into the growing edge (Umwachsungsrand) and 

 the blastopore edge (Urmundrand) in both Elasmobranchii, and 

 Sauropsida. He altogether denies the existence of such a thing as the 

 yolk-blastopore. He says ('92, />. Ü6') : — Balfour hat diesem Piing 

 den Xamen Dottterblastoporus beigelegt und in ihm einen Theil des 

 Ürmunds erblickt. Wie schon früher gesagt, halte ich diese in der 

 Literatur weit verbreitete Ansicht für eine irrige. Denn erstens liegt 

 am sogenannten " Dotterblastoporus " überhaupt keine Oetfnung vor, 



* It is often stated that the neurenteric canal is a part of the blastopore. Strictly speak- 

 ing, this is not true, for the neurenteric canal is a transformation of the posterior part of the 

 archenteron {Contrib. IV.). The dor.'ial opcnUui of the neurenteric canal is however, a part of 

 the blastopore. 



