114 THE FKOO. 



Anteriorly, the two neural folds are connected by a trans- 

 verse fold (Fig. 57), which Yuns across the anterior end of the 

 neural plate, and slightly raises it above the level of the sur- 

 rounding parts ; while at their hinder ends the two neural folds 

 are continuous with the lateral lips of the blastopore. 



The neural folds rapidly increase in height and thickness : 

 the groove between them deepens ; and the folds, becoming more 

 and more prominent (Figs, 58, 59), approach each other, and 

 finally meet in the median plane and fuse together, converting 

 the neural groove into a tube. 



The neural folds first meet about the junction of the head 



FIG. 58. Stages in the early development of the Frog Embryo, seen obliquely 

 from the hinder end. (From a series of wax models by Dr. F. Ziegler, of 

 Freiburg i/B.) 



A, stage in widely the blastopore is nearly circular, and is occupied by the white 

 yolk-plug. B, stage in which the lateral lips of the blastopore have met and fused to 

 form the primitive streak ; the short vertical line, corresponding to the position of the 

 blastopore in A, is the primitive groove ; the depression at the upper end of the primi- 

 tive groove is the greatly reduced blastopore, and the depression at the lower end of the 

 primitive groove is the commencing proetodaeal or anal invagination. Above the blasto- 

 pore is seen the commencing neural groove, bordered by the neural folds. C, later stage, 

 in which the neural groove has deepened, while the neural folds are more prominent and 

 are growing inwards to meet each other. D, stage in which the neural folds have met 

 and the tail is commencing to form. Both blastopore and proctodwum are still present. 

 E, later stage, in which the neural tube is completed and the tail has increased in size. 

 The blastopore has finally closed, and the black spot below the tail is the proctoda?um. 



and neck of the embryo ; and from this point the fusion extends 

 rapidly backwards, and more slowly forwards. The point at 

 which fusion last occurs is a little distance behind the anterior 

 end of the neural tube, at the spot where the pineal body is 

 formed later, the part of the tube in front of this point being 

 roofed in by growth backwards of the anterior or transverse 

 neural fold seen in Fig. 57. 



The neural groove extends back as far as the blastopore 

 (Figs. 57 and 58, B), and the neural folds, as noticed above, 

 become continuous at their hinder ends with the lips of the 

 blastopore. For a short ( time, after completion of the neural 

 tube, the blastopore still remains open, communicating, as seen 

 in Fig. 60, both with the mesenteron and with the cavity of the 



