1905.] Observations on the Development of Ornithorhynchus. 315- 



tissue. Of the three definite postfacial neuromeres the 1st or most anterior 

 has no recognisable ganglionic connection at the stages examined. The 2nd 

 and 3rd postfacial neuromeres are connected laterally with the vago-glosso^ 

 pharyngeal ganglionic plate. Of the five prefacial neuromeres, the 1st, i.c. r 

 the one immediately in front of the facial neuromere, has at no time any 

 ganglionic connection, and lies opposite the hiatus between the trigeminal 

 and the acustico-facial ganglionic plates. The next succeeding neuromeres 

 in front of the 1st, i.e., the 2nd to the 5th prefacial neuromeres, are each 

 connected laterally with the broad basal portion, or wide " pedicle," of the 

 trigeminal ganglionic plate. 



The following points of more special interest amongst those set forth in the 

 extended paper may here be specified. 



(a) The very early differentiation of the layer of yolk-entoderm surrounding 

 the yolk-mass of the monotreme egg. 



(6) The original entire independence of the primitive streak from the 

 primitive knot and its " gastrulation-cavity." 



(c) The subsequent intimate approximation of these structures. 



(d) The early appearance of an area of special differentiation in the 

 vicinity of the primitive streak, in the early blastoderm ; and the later- 

 conversion of this " primitive-streak-area " into an " embryonic area " proper, 

 by the annexation of the region surrounding the " primitive " or " archenteric " 

 knot. 



(c) The precise mode of disappearance of the ventral wall, or floor, of the 

 archenteric or invagination-cavity. 



(/) The occurrence of peculiar segmental cell-masses in the substance of 

 the " primitive knot," where that constitutes the parietes of an archenteric 

 canal or its representative. 



(g) The diagrammatically clear demonstration of various features of neural 

 development, including : the well-marked neuromeric segmentation of the- 

 cephalic region of the flattened medullary plate ; the differentiation of early 

 plate-like ganglionic expansions of the neural crest in the cephalic region ; 

 the presence of various cellular connections between the cephalic ganglionic 

 plates and certain of the neuromeric segments of the medullary plate. 



(h) The relative insignificance of the ' " archencephalic " subdivision of the 

 cephalic portion of the medullary plate, from which the fore-brain and most,, 

 if not all, of the mid-brain, are derived. 



