EARLY DEVELOPMENT OF THE COMMON NEWT. 451 



dilatation. It is either absent or very transitory in Bombi- 

 nator. As the folds enclose the blastopore, which remains 

 open till a much later period, the sinus gives a communica- 

 tion from the exterior to the alimentary canal. When the 

 sinus closes there is still the communication between the 

 neural and alimentary canals, which has now been observed 

 in so many types (Amphioxus, Accipenser, Elasmobranchii, 

 Bombinator, &c.). The elongation of the embryo becomes 

 very decided, and one surface of it becomes nearly flat ; in 

 Bombinator this is the dorsal surface ; in the Newt it is the 

 ventral, so that the latter is curved over the yolk. This 

 difference is due merely to the larger amount of food-yolk 

 in the egg of the Urodele, and cannot be considered of any 

 great morphological significance. The bearings of the 

 increased quantity of food-yolk will be discussed further on. 



E. 



This stage includes embryos, perhaps not quite so far 

 advanced as the one figured in Gotte's Taf. iii, fig. 50, The 

 closure of the medullary folds is now complete throughout, 

 and the vesicles of the brain are obscurely marked. The 

 cranial flexure is already decided, and the whole embryo is 

 somewhat curved upon itself, causing the ventral surface 

 to assume a concave outline (except posteriorly, where the large 

 mass of yolk produces a bulge). A trace of the opening of 

 the sinus is still apparent. 



F (Taf. iii, fig. 52). 

 The ventral curvature now becomes stronger, as does also the 

 cranial flexure. The curvature is in an opposite direction to 

 that taken by Bombinator. The vesicles of the brain are very 

 distinct, and the optic vesicles which commenced in the last 

 stage are now remarkably large, much more conspicuous than 

 in the Bombinator of corresponding age. Another difference 

 presents itself in the fact that in the latter the optic vesicle 

 is an elongated oval, while in the former it is hemispherical. 

 The rudiments of the fifth and seventh pairs of cranial 

 nerves appear as buds from near the dorsal part of the hind 

 brain, higher up than in Bombinator. A few protovertebrge 

 have been formed. Up to this time there has been little or 

 no increase in absolute size, the changes in form being pro- 

 duced by the elongation and narrowing of the embryo. 



G. 



In this stage the cranial flexure is carried further, and the 

 head, as a whole, has taken a spherical shape, very difieren 



