452 W. B. SCOTT AND HENRY F. OSBORN. 



from the shape assumed by the Batrachian head. The 

 rudiments of the visceral arches appear, aud the tail begins 

 to bud out from the yolk sac as an un segmented mass of 

 mesoblast. The number of somites has increased. 



H (Taf. iii, fig. 53). 

 The elongation of the embryo has now progressed to a 

 very considerable extent. The cerebral hemispheres bud out 

 as an unpaired rudiment from the forebrain. Four visceral 

 arches and three clefts have been formed. The tail has elon- 

 gated somewhat; and is still unsegmented. We have been 

 unable to discover anything of the suckers or horny teeth 

 found in the Batrachian larvae. 



I (Taf. iii, fig. 54). (See also PL XXI, fig. 17). 



This stage exhibits a general advance in development, but 

 the only new feature is the appearance of the involution for 

 the mouth. This is transversely elongated, differing from 

 the mouth involution of Bombinator. The head shows 

 swellings, which correspond in position to those which Gotte 

 has named, respectively, kidney swelling, lateral nerve, 

 seventh and fifth nerves, auditory vesicle, and Gasserian gan- 

 glion ; but, owing to the fact that the curvature is in the 

 opposite direction, these organs are separated by wider inter- 

 vals than in Bombinator. 



We shall have occasion to refer to one or two later stages 

 (k and l), which are marked by general increase in size, 

 the formation of the lens, and the appearance of the external 

 gills. 



Segmentation and Formation of the Layers. 



We have not succeeded in securing a complete series of 

 specimens showing all the stages of segmentation, but from 

 those which we have observed there can be little doubt that 

 it proceeds very much in the same manner as in the Frog. 

 Segmentation is asymmetrical, and this characteristic begins 

 to appear at a very early period. The earliest stage we have 

 seen shows two longitudinal furrows, which cut each other at 

 right angles at the upper part of the egg, and passing down 

 the sides, gradually fade and disappear before reaching the 

 loAver pole. The food-yolk even at this period preponderates 

 in the lower part of the eg^, and thus prevents the yolk- 

 division taking place so rapidly as it does above. These 

 furrows may be compared to two meridians on a globe; the 

 next one (judging from the analogy of the Frog) represents 

 the equatorial furrow in Amphioxus, but, for the reason above 



