Development of tF^e ^abpole. 



By J. W. Gatehouse, F.I.C. 



Part II. 



Plates VII. and VII. 



'^^^j>S=^" 



SERIES of sections taken from a single egg on 

 March 5th is shown on Plate II., as stated in our 

 first article, p. 38. The form of this specimen had 

 by this time become considerably modified from 

 its original spherical shape, a portion represented 

 by Figs. 6 to 13 having become flattened and 

 elongated. The yelk mass also is penetrated by 

 two distinct cavities formed by the invagination of 

 the outer layers seen in Figs, i, 2, 10, 11, and 12. 

 These cavities form respectively the neural canal and the 

 mesenteron, which are connected behind, and together form an 

 external opening, termed the blastopore. This is the only 

 opening into the egg, the whole of the exterior, besides this 

 minute hole, being surrounded by the epiblast. This blastopore 

 appears ultimately to produce the anus, which in its early stage is 

 termed the proctodeum. 



In the axial dorsal line, a portion of the mesoblast now sepa- 

 rates into a gelatinous-looking rod forming the notochord. This 

 notochord, though not permanent in the perfect animal, will be 

 found to exist throughout the greater part of the embryonic life, 

 and at once indicates a theoretical, developmental connection 

 between the frog and the amphioxus, in which the notochord is 

 permanent. 



What the use of this notochord is would be very difficult to 

 say, as it appears to have no permanent connection with any of 

 the essential organs of animal life, and is not concerned in the 

 direct development of the vertebral column as might at first sight 

 be supposed. From this time the egg commences to flatten and 

 elongate, the whole of the internal organs appearing with great 

 rapidity. On March 7, the formation of a true skin or epidermis 

 from the epiblast was distinctly seen to have commenced, and at 



New Series. 



Vol. I. 



