No. 2.] THE DEVELOPMENT OF THE NEWT. 343 



stage (Fig. 42) affords us a good view of the primitive streak 

 at its period of maximum extension (Fig. 62, x, z). The lateral 

 lips have just met in the mid-line, and the fused area (Figs. 42 

 and 62) reaches from a point in front of the neuropore {x) to a 

 point some distance behind the anus {p), the only breaks in 

 its continuity being the small perforations of neuropore and 

 anus. If we adhere to our determination to designate the 

 whole extent of fused area as primitive streak, then there is 

 obviously a portion of the streak that lies in front of the 

 neuropore and another portion that lies behind the anus. 

 There is, however, an important consideration that tends to 

 qualify the acceptance of such a view. This is the question 

 of the thickness of the fused blastopore rim, — the germ-ring. 

 For it is obvious that a portion of this fused mass will in any 

 event lie in front of the neuropore, and, unless it exceeds in 

 extent the thickness of the original germ-ring at this point, 

 cannot be strictly regarded as formed by fusion of the lateral 

 lips. Now comparison of Fig. 62 with Figs. 52 and 55 shows 

 that this is actually the case, that the fused area in front of 

 the neuropore is no greater than the thickness of the germ- 

 ring, and that therefore the neuropore in this instance probably 

 lies at the anterior end of the blastopore. If we apply the 

 same test to the fused area behind the anus {cf. the lower lip 

 in Fig. 62 with Figs. 52 and 55) we discover that the thick- 

 ness is considerably greater than the thickness of the original 

 blastopore rim, and we must therefore conclude that the anus 

 does not lie at the posterior end of the original blastopore, but 

 at some distance in advance of it. These facts come out still 

 more clearly on comparison of the thickness of the lateral lips 

 (Fig. 63) with that of the anterior and posterior lips (Fig. 62). 

 The primitive streak, then, is that fused mass formed by the 

 median meeting of the lateral lips plus the thickness of the 

 anterior and posterior lips of the germ-ring.^ 



The early fusion of layers, both anteriorly and posteriorly, 



1 In making this statement I do not wish to be understood as implying a con- 

 stant thickness for the germ-ring from the moment of its first inception. On the 

 the contrary I believe that cell proliferation usually modifies the original thickness, 

 although to a comparatively slight extent. 



