258 
JUClIAiiD ASSllETOX. 
Duval’s 'Atlas/ figs. 161 and 182, and Asslieton, ‘ Quart. Journ. 
Micr. Sei./ vol. 37, PL 20, figs. 4a or 5 and 7 or 8. 
Why is it not an expansion within the area itself in 
Platypus just as it is in the rabbit or bird ? 
What reason is there to suppose that there is in Platypus an 
extension forwards in some quite original manner so as to 
include a spot some distance anterior to it, except for the 
difficulty the authors felt in explaining this spot? 
I would therefoi'e submit to them the above suggestion 
as a defensible alternative explanation. And again, the 
archenteric knot is in no wise histologically like the part of 
other vertebrates to which on Wilson and Hill’s hypothesis it 
is supposed to be homologous. 
They had four eggs showing this spot, namely specimens 
D, DD (twins), Q and Y. 
They give photographs of sections through Y and DD, a 
semi-diagrammatic figure in their previous paper of one (of 
the others ?), and also a schematic mesial plane reconstruction 
of Y. 
Now it must be obvious to everyone that the character of 
the dorsal lip of the supposed blastopore has nothing in 
common with the character of the dorsal lip of the blastopore 
of Amphioxus, or Triton, where it is concerned with gastrula- 
tion by invagination, or Raua or Ceratodus or Scyllium, where 
it is also concerned with gastrulation but less obviously with 
invagination, nor even with the anterior margin of the 
" mesodermsackchen ” in any reptile, bird or mammal (where 
it has nothing to do with gastrulation) either. 
In all these it is a solid lip of more or less columnar actively 
dividing cells, making a compact thickened epithelium of 
very even and regular character, e.g. figs. 417, 418, 419, 
Hertwig’s ' Handbuch,’ vol. i. 
In this Platypus ‘'primitive knot” we see from fig. 2, 
Wilson and Hill, 1903, quite a different nature of epithelium. 
The so-called " dorsal lip ” is an inactive layer of thin 
squamous cells passing into an uneven sac utterly unlike any 
other dorsal lip hitherto described. 
