FATE OF BLASTOPOKE, ETC., IN CHELOXIA. 37 



In this is iiiven a series of transverse sections throno-h the resion of 

 tlie yolk-phig in the Tnonijx embryo represented in Fig. 18 (PJ. III.). 



a has the medullary canal wider than in the part anterior 

 to it, showing that although the siims rlwmhoidalis has 

 been obliterated from the surface-view (compare Fig. IS 

 with Fig. 17) it still exists within. The medullary chord 

 and the cZ/o/yÎ^ï -hypoblast show already a tendency to 

 merge into each other. The yolk-plug appears in the 

 medullary canal as a mass of cells resembling in shape 

 the odontoid process of the axis. A comparison 

 with Series I, /', shows that the yolk-plug has greatly 

 diminished in size. The fact that the part of it lying 

 within the medullary cavity is larger and more rounded 

 than the part embraced between the edges of the medul- 

 lary folds shows that the approachment of the medullary 

 folds towards the median line has compressed the yolk- 

 plug laterally. Note that the karyokinetic figures are 

 numerous in the walls of the medullary-, and chorda-, 

 canals in tins and succeeding sections, showing that this 

 is the seat of active growth. 



Anterior to this section, there are two in which the 

 front termination of the yolk-plug is seen as a mass much 

 smaller tlian that in a. 



b is the section next behind a. The medullary-, and the 

 cliorda-, canals approach each other and the fusion of 

 their wall is greater. 



C is next behind h. Here, we are in the region of the 

 neurenteric canal, i.e., the medullary canal has united with 

 the (,'/;or(k-canal. Owing to the slight ol:»liquity of the 

 plane of the section, there is on the left side (right, in the 

 figure) of the neurenteric canal a ledge of cells projecting 

 transversely into the middle of the canal. Comparing 

 this section with Series I. /, we perceive that the upper 

 surface of the yolk-plug was in the latter section at the 



