«0 ADAM SEDGWICK. 



In other Arthropoda in which a primitive streak is present, 

 its position with regard to the blastopore cannot be determined ; 

 because the blastopore is not present in those cases in which 

 there is a primitive streak. 



With regard to the two first cases the blastopore of the 

 Vertebrata closes, and the anus is subsequently (very late) 

 formed within the area of the primitive streak. 



In Peripatus, however, the hinder division of the blastopore 

 does not close but travels slowly back over the area occupied 

 by the primitive streak to its position at the hind end of the 

 body. 



I may here mention a fact which I observed last summer in 

 the newt (Triton cristatus). In this animal the blastopore 

 appears not to close but to persist as the anus. This state- 

 ment is based on surface views of a large number of embryos 

 from the stage when the egg is round until hatching. In all 

 these stages I never saw an embryo without an opening at the 

 hind end of its body. I very much regret that I have not had 

 time to confirm this observation by means of sections. 



If true it is most interesting as being the only known case 

 in which the blastopore of Vertebrata actually persists as 

 the anus. 



In the case of larvae which leave the egg at an early stage 

 of development, no primitive streak is developed, but the 

 mesoblast partly grows in from the lips of the blastopore, and 

 partly arises as mesenchyme. 



In Amphioxus fourteen pairs of somites are derived as 

 hypoblastic pouches, the remainder are formed from hypoblastic 

 tissue, the exact behaviour of which is not explained by 

 Hatschek. 



In those Vertebrata with primitive streak, the anterior 

 somites may be regarded as arising from hypoblastic meso- 

 blast ; but the greater part are formed from primitive streak 

 mesoblast. 



In Peripatus, the mesoblast arises behind the blastopore from 

 the primitive streak, and grows forward as two bands, exactly 

 as in worms ; but it arises from a primitive streak. 



