No. 2.] THE DEVELOPMENT OF THE i\EWT. 341 



absolute, determined by the extent of antero-posterior coa- 

 lescence. The longest space that I have observed between, 

 the two openings is represented in Fig. 42, and it must be 

 noticed that this extreme case affords no support to the view 

 that there is an elongation of a circular blastopore. In such 

 an Q.gg as that shown in Fig. 39 the other extreme is indi- 

 cated ; in this instance the distance separating neuropore and 

 anus would obviously be slight. 



The neuropore is quite transitory, remaining open but a few 

 hours, and closing long before the meeting of the medullary 

 folds. The position of the neuropore is more constant than 

 that of the anus ; it always lies near the anterior end of the 

 blastopore, and the fused area in front of the neuropore is, in 

 all specimens I have examined, less than the fused area be- 

 hind the anus. Fig. 6^ represents a horizontal section through 

 Fig. 40, in which the blastopore has narrowed to a longi- 

 tudinal slit. Fig. 62 shows a sagittal section through a stage 

 similar to that shown in Fig. 42. The neuropore lies dorsally 

 (anteriorly) and the anus below, and between these is shown, 

 the mass of ingrowing lateral cells which have just met in 

 the mid-line. Regarding the significance of this strangely 

 persistent ancestral structure, conjecture is at present idle. 

 We are not even, I think, able to determine whether this 

 anterior portion of the blastopore primitively opened freely to 

 the exterior as the neuropore, and has been only secondarily 

 caught in as the "neurenteric canal" by the precocious closing 

 of the medullary folds, or whether the reverse is true. 



The posterior opening remains as the permanent anus. 

 The backward extension of the medullary folds brings the 

 anus to lie just behind them (Fig. 43). The external appear- 

 ances do not differ from those pictured by Ziegler ('92) for 

 the frog, while sections show that this opening in the newt 

 maintains its integrity throughout, and eventually functions 

 as the anus. The sections figured by Miss Johnson ('84) ade- 

 quately represent in this respect the conditions I have found 

 in the later stages of the newt. 



Pi'imitive Streak. — The term primitive streak, originally 

 applied to a certain darkened region in the posterior part of 



