72 



GENERAL DEVELOPMENT. 



Fig. 40. Tkansverse section through 



PART OF AN EMBRYO OF PeTROMYZON PlANERI 

 OF 2-36 HOURS. 



m.c. medullary cord; ch. notochord; ah 

 alimentary caual; ms. mesoblastic plate. 



General history of the development. Up to about the time when 

 the enclosure of the hypoblast by the epiblast is completed, no 



external traces are visible of any 

 -^ ^ of the organs of the embryo ; 



but about this time, i.e. about 

 180 hours after impregnation, 

 the rudiment of the medullary 

 plate becomes established, as a 

 linear streak extending forwards 

 from the blastopore over fully 

 one half the circumference of 

 the embryo. The medullary 

 plate first contains a shallow 

 median groove, but it is con- 

 verted into the medullary cord, 

 not in the usual vertebrate 

 fashion, but, as first shewn by Calberla, in a manner much more 

 closely resembling the formation of the medullary cord in Teleostei. 

 Along the line of the median groove the epiblast becomes thickened 

 and forms a kind of keel projecting inwards towards the hypoblast 

 (fig. 89, nc). This keel is the rudiment of the medullary cord. It 

 soon becomes more prominent, the median groove in it disappears, and 

 it becomes separated from the epiblast as a solid cord (fig. 40, mc). 



By this time the whole embryo has become more elongated, and 

 on the dorsal surface is placed a ridge formed by the projection of the 

 medullary cord. At the lip of the blastopore the medullary cord is 

 continuous with the hypoblast, thus forming the rudiment of a neur- 

 enteric canal. 



Calberla gives a similar account of the formation of the neural canal^ 

 to that which he gives for the Teleostei (vide p. 59). 



He states that the epiblast becomes divided into two layers, of which 

 the outer is involuted into the neural cord, a median slit in the involution 

 representing the neural groove. The eventual neural canal is stated to 

 be lined by the involuted cells. Scott (N"o. 87) fully confirms Calberla on 

 this point, and, although my own sections do not clearly shew an involu- 

 ti(m of the outer layer of epiblast cells, the testimony of these two 

 observers must no doubt be accepted on this point. 



Shortly after the complete establishment of the neural cord the 

 elongation of the embryo proceeds with great rapidity. The pro- 

 cesses in this growth are shewn in fig. 41, A, B, and C. The cephalic 

 portion (A, c) first becomes distinct, forming an anterior protuberance 

 free from yolk. About the time it is formed the mesoblastic plates 

 begin to be divided into somites, but the embryo is so opaque that 

 this process can only be studied in sections. Shortly afterwards an 

 axial lumen appears in the centre of the neural cord, in the same 

 manner as in Teleostei. 



The general elongation of the embryo continues rapidly, and, as 

 shewn in my figures, the anterior end is applied to the ventral 



