354 ORR. [Vol. I. 



about the same thickness as the general epiblast. Agaia 

 (Comp. Embryol.), he has pictured the chick at eighteen hours 

 with an epiblast of a single layer of cells, which only on 

 each side of the median longitudinal portion is thickened to 

 a double layer. In his paper, to which I have referred in 

 the introduction, Balfour represents a somewhat similar con- 

 dition as existing in Lacerta muralis, as does also Weldoji. 

 Scott and Osborn ^ figure the same in the newt, and I have 

 observed it in Amblystoma. The most striking illustration, 

 however, is that of Bombinator igneus, which is also very 

 carefully described by Goette? Few other authors have re- 

 ferred to this appearance in their text. It has apparently 

 been usually considered that the medullary groove and canal 

 were at this stage the primary feature, to which the nature 

 of the walls was of secondary importance ; yet it is more 

 natural to consider the walls as primary and the lumen as 

 incidental. Goctte describes a double symmetrical rudiment 

 of the nervous system. In the figure of the round cross- 

 section of the embryo, the lateral halves of the rudiment ap- 

 pear as lens-shaped thickenings of the " nervous layer " of 

 the epiblast. These thickenings are separated by a median 

 portion of exactly the same appearance as the parts of the 

 nervous layer adjoining the lateral edges of the thickenings. 

 In the head the paired thickenings separate farther laterally, to 

 unite as a transverse curved thickening in the most anterior 

 part of the medullary plate. I give here, in his own words, 

 Coette's explanation of this appearance : — 



" Es lasst sich alsdann nicht verkennen, das der vom Axen- 

 strange auf das obere Keimblatt ausgelibte Druck die Ursache 

 fiir die urspriingliche bilaterale Anordnung der Anschwellung 

 desselben in ihrem mittleren Abschnitte ist. Aus der folgenden 

 Entwickelung ergibt sich aber, dass damit keine wirkliche 

 Doppelanlage im oberen Keimblatte gegeben ist. Dcnn indem 

 jene Seitentheile auf Kosten der iibrigen Ausbreitung der 

 Grundschicht deutlicher anschwellen, nimmt dass sie liber dem 

 Axenstrange verbindende Mittelstiick im Verhaltniss zu jenen 

 diinnen peripherischen Theilen an Machtigkeit zu, offenbart 



' Scott and Osborn. — On some Points in the Early Development of the Common 

 Newt. — Quart. Jour, of Mic. Set., Vol. XIX., N.S., 1879. 



* Goette. — Entwickelungsgeschichte der Unke. Bombinator Igneus. 



