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iProf. J. T. Wilson and Mr. J. P. Hill. 



[Jan. 21, 



layer is thickened and . cubical, and within which a mesodermal sheet 

 has already undergone a wide extension. 



It is peculiarly unfortunate that, owing partly to the difficulty of 

 dealing with the delicate and originally collapsed and torn vesicular 

 wall after the cutting open of the egg, partly to the failure to detect 

 any visible differentiation apart from the knot, and our consequent 

 conviction that the latter was the sole trace of embryonic organisation 

 yet present, the orientation of the distinct portions into which the 

 vesicle was separated was not adequately determined and preserved. 

 It thus became impossible for us to guarantee that the planes of 

 sectioning of the remaining portions should be accurately co-ordinated 

 with each other, or with that of the important piece first sectioned. 

 This failure, not wholly blameworthy, when the conditions of the task 

 are appreciated, has introduced an element of conjecture into our sub- 

 sequent attempt to determine the precise relation to the knot of the 

 primitive streak area, of whose existence we later on became aware. 

 Nevertheless, we think that we shall be able to establish these rela- 

 tions with at least a high degree of probability. 



In this preliminary paper we propose to restrict ourselves to an 

 account of the highly interesting area which first attracted our atten- 

 tion, and which we have already referred to as a "primitive knot." 

 It will, we think, be admitted that no possible doubt can be entertained 

 of the justice of employing for its designation a term which would 

 stamp it as the homologue of the well-known structure in many 

 Sauropsida to which the same name has been applied. It is a true 

 primitive or gastrula knot, in the Sauropsidan sense, possessing a 

 transversely elongated gastrula-mouth or blastopore and an invagina- 

 tion cavity, which both in appearance and minute structure resembles 

 in the closest manner the structure described under these names in 

 various Reptilian forms. 



But if this be admitted, we come face to face with this extraordi- 

 nary fact that, in addition to this reptilian-like gastrula, there is to 

 be found in Ornithorhynchus, quite distinct and even remote from 

 this gastrula knot, a region in which there is being differentiated 

 independently a primitive streak of quite ordinary and typical 

 mammalian character. 



The oblong portion of the Avail of the vesicle containing the 

 primitive knot near its centre, and measuring about 8 x 10 mm., 

 was left adhering to the portion of the shell covering it. The direct 

 observation and photography of the piece were thus confined to its 

 deep aspect so as to avoid undue disturbance and injury. The 

 whole piece was then double imbedded in photoxylin and paraffin 

 and cut into serial sections 10 micra thick. The sections were stained 

 in hsematein and eosin. 



The plane of section chosen was approximately at right angles to 



