No. 2.] THE DEVELOPMErNT OF THE NEWT. 339 



considered as ''closing" in both directions. This I think is 

 what usually happens in the newt. 



The lateral cells continue to press in towards the mid-line 

 until the blastopore exists only in the form of a longitudinal 

 slit. This at first opens into the archenteron along its whole 

 length, but soon the ingrowing lateral cells meet along the 

 middle of the slit, leaving a small circular orifice at each end. 

 The anterior of these openings is the evanescent neuropore; 

 the posterior remains open as the permanent anus (Fig. 42). 



It is evident from the facts above adduced that very great 

 significance cannot be attached to the manner of closure of the 

 blastopore. The work of other observers points to the same 

 conclusion. The blastopore in amphibia, and in other verte- 

 brates also, has been described as closing both from behind for- 

 wards and from before backwards, as well as in both directions 

 simultaneously. The closing of the Urodelan blastopore in the 

 middle has been observed by Schanz ('87) and Morgan (-89), and 

 is a constant occurrence in the newt. Whether this mesial 

 coalescence is in all cases preceded by a coalescence at either 

 end of the blastopore varying in character and extent, or 

 whether the mesial coalescence sometimes steps in directly upon 

 the cessation of the uniform convergence of the marginal cells 

 seems to me an open question. I am convinced that in the 

 newt, at least, occasional variations cover several of the 

 possibilities of antero-posterior coalescence. The mesial ap- 

 proximation of the lateral lips, resulting first in the formation 

 of a longitudinal slit and then of an anterior neuropore and a 

 posterior anus seems invariably to follow the diversity of the 

 earlier conditions. 



Variation in the manner of blastopore closure in the same 

 group of animals, or even in the same species, is not uncommon. 

 E. B. Wilson ('89) observed in Limibriciis " a considerable 

 variation in the closure of the blastopore, owing to differences 

 in the rate of folding between the sides and the posterior 

 margin of the blastopore. As a rule, the sides fold in more 

 rapidly than the hinder lip, thus giving rise to a slit-like blasto- 

 pore, but in some cases the reverse is true, so that the blasto- 

 pore never appears as a slit, but always as a rounded opening" 



