1903.] 



Primitive Knot, etc., in Ornittiorhynchus. 



321 



front without change of character as far as the transversely elongated 

 blastopore seen in fig. 1 and in section in fig. 2. Thence it is con- 

 tinued round the anterior lip of the blastopore so as to line the roof of 

 the blastoporic (archenteric) canal or gastrula-cavity. 



Posteriorly to the knot the thin ectoderm is continued forwards, 

 but on reaching the posterior edge of the knot, its cells lose their 

 markedly flattened character, and become continuous with a mass of 

 cuboidal or even rounded cells, which forms the prominence of the 

 posterior lip of the blastopore. The superficial cells of this mass do 

 not, however, wholly surrender their epithelial arrangement, and are 

 continued forwards in the hinder wall and floor of the gastrula-cavity 

 to complete the cellular lining of this cavity. This cellular lining 

 consists, for the greater part of its extent, of large and cuboidal cells, 

 but in the hinder part of the roof, and extending from this upwards 

 around the anterior lip of the blastopore, the cells are more flattened, 

 ultimately becoming continuous with the thin ectoderm overlying the 

 anterior part of the knot. 



Figs. 1 and 2 together enable one to realise the form of the gastrula- 

 cavity as a wide, but dorso-ventrally flattened cavity opening 

 posteriorly by a transversely-elongated blastoporic aperture. 



The posterior lip of the blastopore is formed by a mass of cells 

 which, if not wholly indifferent, at least show a less clear differentia- 

 tion into superficial and deep than is found elsewhere. 



As was indicated at the outset, the resemblance between the 

 primitive knot in Ornithorhynchus and that found in a number of 

 reptilian forms is very striking. We would particularly draw attention 

 to the figures by Mitsukuri* of the primitive knot in Chelonia, and 

 more especially to his figs. 9 and 13 on Plate 8, which show a 

 remarkable similarity to the condition here described, in all essential 

 features. In Ornithorhynchus we lack entirely the columnar arrange- 

 ment of the ectoderm over the knot, and the anterior lip of the blasto- 

 pore there shown, otherwise the characters both of the knot and of (.he 

 gastrula-cavity and its wall appear to be almost identical with those 

 figured, especially in Mitsukuri's fig. 9, Plate 8. 



In a future more extended communication we hope to illustrate more 

 adecruately the points above set forth, and in addition to add something 

 in the way of elucidation of the fate of the primitive knot in somewhat 

 later stages. We propose also to describe and illustrate the con- 

 dition of the primitive streak area, which at the period now dealt with 

 already co-exists with, though independently of, the primitive knot, 

 but which later by extension comes into more intimate relationship 

 with the knot. 



* K. Mitsukuri, " On the Process of Grastrulation ill the Chelonia," 4 Journ. Coll. 

 Sci., Japan,' vol. G. Cf. also Gertrude C. Davenport, ' Eadcliiffe Coll. Monograph,' 

 tfo. 8. Boston, 1890. Dendy, ' Qu. Jl. Micro., Sci.,' vol. 42, p. 18, 1899. 

 VOL. LXXI. 2 A 



