DEVELOPMENT OF VERTEBRATES. 



would have been found to be the case with Amphioxus had the 

 stage a little older than that on Plate 5, A III, been represented. 

 The cavity marked (a I}, as was the case with Amphioxus, is that 

 of the alimentary canal. 



The similarities between the mode of formation of the hypo- 

 blast and alimentary canal in this animal and in Amphioxus are 

 so striking and the differences between the two cases so slight 

 that no further elucidation is required. One or two points need 

 to be spoken of in order to illustrate what occurs in the Frog. 

 When the involution to form the alimentary canal occurs, certain 

 of the lower layer cells (marked Jiy) become distinguished from 

 the remainder of the lower layer cells as a separate layer and 

 form the hypoblast which lines the alimentary canal. It is to 

 be noticed that the cells which form the ventral epithelium of 

 the alimentary canal are not so soon to be distinguished from 

 the other lower layer cells as those which form its dorsal epi- 

 thelium. This is probably a consequence of the more active 

 growth, indicated by the asymmetry of the involution, on the 

 dorsal side, and is a fact with important bearings in the ova 

 with more food material. The cells marked in and coloured 

 red also become distinguished as a separate layer from the 

 remainder of the hypoblast and form the mesoblast. The re- 

 mainder of the lower layer cells form a mass equivalent to the 

 yolk-sac of many vertebrates, and are not converted directly into 

 the tissues of the a'nimal. 



Another point to be noticed is the different relation of epi- 

 blast cells to the hypoblast cells at the upper and lower side 

 of the mouth of the involution. Above it, on its dorsal side, the 

 epiblast and hypoblast are continuous with one another. On 

 its ventral side they are primitively not so continuous. This 

 is due to the epiblast, as was before mentioned, 'growing round 

 the lower layer cells on the ventral side, vide B n, and merely 

 remaining continuous with them on the dorsal. The importance 

 of these two points will appear when we come to speak of other 

 vertebrates. 



The next animal whose development it is necessary to speak 

 of is the Frog, and its differences from the mode of development 

 are quite easy to follow and interpret. Segmentation is again 

 not uniform, and results in the formation of an upper layer of 



