398 EYCLESHYMER. [Vol. X. 



12. The Olfactory and Oral Grooves in Rana. 



In PI. XXII, Fig. 6, an embryo of 70 hrs. is represented. 

 The neural folds are anteriorly wide apart. At this stage 

 paired grooves {ol.g) arise in the cephalic portion of the neural 

 fold; these grooves are short, shallow and deeply pigmented. 



A later stage is represented in Fig. 7 in which the neural 

 folds are united along their entire length. The distal ends of 

 the grooves have extended antero-laterally, while the proximal 

 ends have been united by the closure of the neural folds caus- 

 ing the V-shaped arrangement shown in the drawing. 



In Fig. 8, the grooves have reached their maximum exten- 

 sion, having grown antero-laterally until their distal ends 

 reach well over toward the anterior border of the primary fore- 

 brain. At this time the pits form at the distal ends of the 

 grooves and the nose becomes localized. The lines of pigment 

 are exactly similar to, and continuous with, the line which 

 indicates the suture formed by the closure of the neural folds. 



The mouth usually arises as an independent median invagi- 

 nation; in some cases, however, it shows extremely interesting 

 peculiarities. 



In the development of the nervous system of Rana the 

 closure of the neural folds occurs in two ways; the most fre- 

 quent is by a median approximation of the lateral portions of 

 the neural folds, together with an antero-posterior folding of 

 the cephalic portion. Often the antero-posterior folding does 

 not occur, or if so, to a slight extent only. When this occurs a 

 very shallow pigmented groove is formed just at the anterior 

 end of the neural groove. 



At a time shortly after the folds have begun to approach 

 posteriorly the appearance of the oval groove is foretold by a 

 very slight depression at the anterior end of the neural groove. 

 This would appear in a somewhat earlier stage than Fig. 6. 

 With the progressive closure the groove extends farther ante- 

 riorly (Figs. 7 and 8); in the latter it reaches just beyond the 

 region of the primary fore-brain. The oval invagination is 

 now outlined and localized in the distal end of the groove in a 

 manner quite similar to the olfactory pits. 



