314 Prof. J. T. Wilson and Dr. J. P. Hill. [Dec. 5, 



remote from, the primitive or archenteric knot — a primitive streak, of 

 typically mammalian character. 



(6) A stage which, for convenience of classification, is termed by the 

 authors the " postgastrular." This is characterised, inter alia, by the presence 

 of a greatly elongated area of axial differentiation consisting of a primitive 

 streak, together with a " Hensen's knot " and a long " archenteric plate," the 

 two latter structures being derivatives of the " primitive " or " archenteric " 

 knot of the preceding period. The archenteric plate ends anteriorly in an 

 expanded " protochordal-plate " segment. 



In this stage the primitive streak has practically retained its earlier form 

 and dimensions, but it is now contiguous to, and has the appearance of being 

 continuous with, the posterior end of the thickening of " Hensen's " 

 (archenteric) knot. 



The " archenteric plate " is derived from the dorsal wall of a greatly 

 elongated archenteric canal (i.e., the archenteric invagination-cavity of the 

 preceding stage). From this archenteric plate are derived both the chorda- 

 Anlage and also, laterally to this, a small amount of " gastral " mesoderm. 



(7) A "neurular" stage is described, in which the medullary plate is 

 present as a completely flattened and spatulate structure. This is expanded, 

 in the head-region, into a wider " cephalic plate." This again is divisible into 

 two divisions which seem to correspond to the " archencephalon " and the 

 " deuteroencephalon," respectively, of von Kupffer. 



The cephalic plate further exhibits a series of neuromeric segments, all of 

 which, with the exception of the most anterior, belong to the region of the 

 " deuteroencephalon." One neuromere, on account of its relations to the 

 acustico-facial ganglion, is designated as "facial." In front of this neuromere- 

 at least five " prefacial " neural segments are met with, whilst behind the 

 same "facial" neuromere three definite "postfacial" neuromeres are en- 

 countered. 



In the posterior region of the cephalic portion of the medullary plate, 

 indications are to be found, in the case of slightly younger specimens, of the 

 existence of additional " postfacial " segments behind the persistent 3rd post- 

 facial neuromere. 



Laterally from the margins of the cephalic medullary plate there are 

 found, on each side, three ganglionic plate-like expansions of the "neural 

 crest." These represent the rudiments of the trigeminal, acustico-facial, and 

 vago-glossopharyngeal ganglia. 



The relations of these ganglionic plates with the various neuromeres are as 

 follows : — The facial neuromere is connected with the " pedicle " of the 

 acustico-facial ganglion, which at this period consists of a tract of cellular- 



