240 EICHAED ASSHETON. 



Morgan and Ume Tsuda conclude that all except " the 

 thickness of the medullary folds ^' round tlie dorsal lip of the 

 blastopore is formed by the growth of the lip. 



Roux shows the same in his figure. 



Pfliiger, however^ thinks it possible that a considerable 

 length of the anterior part of the nervous system is formed in 

 this black hemisphere, and with Pfliiger I quite agree on this 

 point. 



I find that the neural plate in normal embryos at the time 

 it becomes visible on the surface extends through fully 170°, if 

 not more, while the distance through which the dorsal lip of 

 the blastopore travels I cannot make out to be more than 70° 

 at the most ; that is, from a spot a little below the equator to 

 the lower pole, or perhaps a little beyond it. 



Figs. 7 to 14 represent diagrammatically the views put 

 forward in this paper. Fig. 7 is the fully segmented frog's 

 egg, the white pole placed to the right of the paper ; the black 

 pole or roof of the segmentation is placed to the left, as repre- 

 senting the future anterior end of the embryo. All the others, 

 8 — 14, are arranged similarly. Fig. 8 represents the stage at 

 which the dorsal lip of the blastopore has become established. 



Up till now there has been but one general centre of growth. 

 From this moment the secondary centre of growth is in 

 existence, and we have now the commencement of the con- 

 version of an embryo radially symmetrical into an embryo bi- 

 laterally symmetrical. As yet only the dorsal part of this 

 secondary area of proliferation is in existence, and accordingly 

 the dorsal part of the embryo is developed more rapidly than the 

 ventral, as the annexed figure 9 shows. In this figure the 

 ventral part of the secondary area has just been completed, and 

 now the whole of the secondary area of proliferation, the homo- 

 logue of the whole of the primitive streak of the rabbit, is 

 complete, and new material is added to ventral and lateral 

 and dorsal parts of the embryo, as diagram fig. 10 illustrates. 

 The shape now rapidly changes, and the radial symmetry is 

 lost and the bilateral symmetry acquired, fig. 11. 



The neural plate is indicated in fig. 11 by the continuous 



