70 ARTHUR DKNDY. 



liind limbs (cf. fig. 98,), but at Stage R this spiral has become 

 to a great extent straightened out, and the hinder part of the 

 tail lies against the animal's right side (fig. 104). 



The cranial flexure takes place in the usual way, commenc- 

 ing at Stage J, when the front part of the head, including the 

 fore-brain, is seen to be bent down at right angles (fig. 57). 

 Later on the whole of the front half of the body becomes 

 curved ventrally in a spiral, best seen about Stages M and N 

 (figs. 92, 94), while in the lumbar region there is a slight 

 curvature in the opposite direction (figs. 83, 84, 92j 94). By 

 the time Stage R is reached the curvatures of the body have 

 to a large extent disappeared, and the cranial flexure has 

 straightened out, the head acquiring a marked Chelonian 

 aspect, with a conspicuous " shell-breaker " on the snout (fig. 

 104). A quite temporary flexure of the head in an upward 

 and backward direction is conspicuous in some of the earlier 

 stages of development, especially Stage G (figs. 48 — 50) ; it is 

 caused apparently by the restraining influence of the pro- 

 amnion, and very soon disappears. 



The limbs make their appearance as outgrowths of the 

 Wolffian ridges at Stage M (fig. 92), and develop in a perfectly 

 normal manner, five digits appearing on each at Stage Q (figs. 

 102, 103). 



The development of the visceral arches, superior maxillary, 

 and fron to-nasal processes appears to be perfectly normal 

 (cf. figs. 92, 94, 99). 



The Central Nervous System. — The development of the 

 central nervous system, so far as investigated, takes place quite 

 in the ordinary manner, except perhaps for the temporary 

 modifications in the form of the brain due to the restraining 

 influence of the pro-amnion. 



Already at Stage C the medullary plate (fig. 9, M. P.) is 

 recognisable as a flattened area in front of the blastopore, 

 where the epiblast cells are prismatic and arranged in two or 

 three layers. At Stage D (figs. 6, 7, 9, 10) the medullary 

 groove appears as an invagination of the cells of the medullary 

 plate in the mid-dorsal line. At Stage E the front end of the 



